Transcript of the lectures given by Professor Ayrton at the London Institute, February and March, 1883:
ELECTRIC LIGHTING AND LOCOMOTION.
Forty-three years ago it was practically shown by Jacobi, on the River Neva, that locomotion by the aid of electricity was possible. No experiment, then, merely to prove this fact is needed at the present day. Nevertheless, both experiments and calculations are still necessary to convince the world that electric locomotion is not only possible but also commercially practicable. That electric transmission of power possesses great advantagesâ€”arising, first, from the small weight of the electromotor per horse-power developed by it; secondly, from the fact that large amounts of power can be transmitted through quite thin flexible wires must also have struck any one who has ever seen a powerful machine driven by electricity. Its disadvantages are generally summed up in the single sentence that it is too expensive. But people, as a rule, do not trouble themselves to inquire whether it is really more expensive than other modes of transmitting power, and whether, if this be the case, it arises from some inherent defect in the principle itself, or merely from some removable imperfection in the maohinery hitherto employed for the purpose.
At the present day the public may be divided into two classesâ€”the one who thinks that electric locomotion and transmission of power will act as a sort of philosopher's stone, and solve all difficulties; the other, that it is a new fangled notion, and carefully to be shunned. Tho first class implores to be allowed to buy shares in any company that can in any sort of way have the word "electric" tacked on to its title, while the other buttons up its pockets at the mention of the word electricity, and busies itself with what it terms "something useful and practical." And, unfortunately, the repeated failures of the sanguine members of the first class to reach the El Dorado they have imagined to themselves only strengthens the belief of the more cautious members of the second class in their shrewdness aud in the wise decision they fancy they have arrived at, that electricity is a mere will-o'-the-wisp.
But you, I am sure, will not have that opinion when you have seen tonight what can be done by electricity. I turn on this electric tap, and at once this fan commences to revolve rapidly, and sends forth a rush of wind. That, perhaps you will say, could as well be done by a belt driven by a steam engine; but I turn on another tap in the same supply wire, and at once the fan is lighted up by this electric lamp. That could not have been done merely by a steam engine. Again, a third tap; this electric fire burns, and the water starts boiling in this glue pot. [Experiment shown.] This thin flexible wire then furnishes a supply of electric power which can at will be used for producing motive power, light, or heat.
Now, to produce this supply of electric power, we must first have a steam engine, a gas engine, a water-wheel, or windmill to produce motion, and a dynamo machine to convert the mechanical energy into electric energy. These may be in the same room, or in another part of the building, or in another street, or away in the country; the electric power may be brought by a wire and used directly, or it may be stored in Faure-Sellon-Volckmar accumulators and used at some subsequent time, as I am doing now, thanks to the courtesy of the Electric Power Storage Company. Indeed, I may mention that in all my experiments I am using only stored electricity; and in this room, where we are utilising this supply of electric power, we must have an electro-motor if we want motion, an electric lamp if we want light, and an electric fire if we want heat. The transmission of power by electricity seems, then, very useful. Can it be made commercially remunerative?
As there are, doubtless, many in this hall who are deeply interested, either from scientific or from commercial reasons, in the answer to this question, and generally in the practical applications of electricity, it will be well to commence this lecture by considering, as shortly and as popularly as possible, what are the real difficulties besetting electric transmission of power, and what are the means electric engineers aro proposing for surmounting them. Let us then attack the question together, not in the rosy, sanguine humour of the promoters of electric bubble companies, nor with the cold, hard scepticism of those who forget that with the early locomotive engines the fireman had to run along the road by the side and stoke, but as common-sense Englishmen who are anxious to know whether at the present moment the pros or the cons are in the majority.
An electric current, as you have seen, can produce heat; but, unfortunately, not only can an electric current produce heat, but an electric current always is producing heat. Now, while it is very proper that heat should be produced in an electric lamp or furnace, it is quite out of place in the dynamo machine or generator of electricity, or in the leading wires, or in the electro-motor, which converts the electric energy into mechanical energy. In all such cases the heat produced by the electric current corresponds with the heat produced by friction in machinery, and causes the same sort of waste of power.
But this loss of power, we shall see, is not necessarilv so very great even when the power has to be transmitted a considerable distance for when there are a number of machines, lamps, or motors that have to be worked by electricity there are two ways in which they may be connected together. They may all be joined in what is technically called "parallel," or they maybe connected in "series." In the first case the current (as was illustrated by diagrams) that passes through any one of these machines does not pass through any other, and the generator must produce a large currentâ€”as large, in fact, as the sum of all the currents passing through all the machines. In the second case, the "series" arrangement, the same current of electricity passes through all the instruments one after the other in succession, and the generator only produces a current equal to that passing through any one of them. The ordinary methods of supplying houses with water or with gas are examples of the parallel system, because the water or gas that is supplied to any one house does not go to any other. The postman brings the letters and the boy leaves penny papers at a number of houses in parallel, while a copy of The Times, lent out at a penny an hour, is furnished to a neighbourhood in series.
And just as a newspaper or a book from a circulating library loses in freshness, although gaining in thumb-marks and dog-ears, when supplied to its readers in series, and as water loses head or pressure as it flows down astream, so electricity loses its potential or power to work in passing through a number of machines worked by it in series.
To work, then, a number of electric motors or lamps in the multiple arc system we must at starting supply a large quantity of eloctricity at, it may be, only a low potential or pressure; whereas, if they are joined in series, the current may be small, but the initial pressure must be great.
In all electric railways and tramways constructed up to the present time the multiple arc system has alone been adopted. This you will see from the photographs that I will now project on the screen of all the most important electric railways yet constructed. First comes three photographs [Photos, on screen] of the small circular railway, 900 yards long, of 3ft. 3in. gauge, laid in the grounds of the Berlin Exhibition in 1879, and exhibited in 1881 at the Crystal Palace, Sydenham; next, a photograph of the Lichterfelde Railway, near Berlin; then the electric tramway in the Champs Elysee, Paris; and that of Charlottenburg, near Spandau. The latest electric railway, the one six miles long, now in course of construction at Portrush, near the Giant's Causeway, in Ireland, I shall reserve, as I shall have much to say about it later on in these lectures.
[Photographs were then exhibited of the various electric railways, which were described in detail]
But the amount of power lost in heat when a certain definite amount of power is being transmitted through a given wire is very different, whether the multiple arc or the series system be adopted, because the heat produced per minute depends on the strength of the current used in the transmission and not in the amount of power transmitted. Indeed, it depends not merely on the current, but on the square of the current, so that if the current be doubled the waste of power becomes four times as great as before. Economy in electric transmission, therefore, requires small current, and, if much power is to be transmitted, small current necessitates high potential and the motors or lamps working in series. Similar considerations apply to the transmission of power by water: to obtain great economy high pressure is absolutely necessary. The accompanying tables, taken from Professor Perry's Cantor Lectures, show the amount of loss of power when different amounts are transmitted different distances by water at an united pressure of 700lb., and at 1,400lb. per square inch, as well as the amount lost when electricity is employed with a difference of potential between the terminals of the dynamo, first of 8,000 volts, secondly of 80,000 volts.
But high electric potential or pressure is generally considered to mean danger to life, and series working of machines or lamps has this disadvantage, that if from any accident the current is stopped passing through one it is stopped passing through all.
The danger arising from the use of high potential can be very much diminished if, instead of using the earth as part of our return circuit, we employ not only an insulated wire to convey the current, but also another insulated wire to bring it back again, and if in addition the current flowing through the whole circuit is not intermittent. This may be shown by the following experiment: Attached to these wires, well insulated from the ground, are the insulated coatings of this battery of charged Leyden jars. Now, although the difference of potentials between the wires is some thousands of volts, far, far greater than is at present used, or proposed to be used, in any system of electric transmission of power, I can, as you see, touch either wire with perfect impunity. But if either of these wires be replaced by a wire in contact with the earth, say replaced by one of the gas pipes, aud I were now to touch the other, I should receive a violent shock.
As, however, my love of electric experiments does not extend to shocks, I will touch the insulated wire by deputy, using this metallic rod, and the result is, you see, a bright spark. [Experiment shown.] Both the going and return wires, therefore, should be insulated.
But, next, the current flowing in them must not be discontinuous if danger is to be avoided. For if, instead of keeping the difference of potentials between any two points in the two insulated wires constant, I allow rapid fluctuations in this difference to take place, then, no matter how well the wires be insulated, a violent succession of shocks will be received on touching either of the wires, even although the other be not touched, so that no circuit in the ordinary sense is completed through the body. To show this experimentally I shall replace this insulated battery of Leyden jars with an induction coil, which, as well as the voltaic cells used in charging it, are thoroughly well insulated from the earth in every part. Let me, however, touch either of these wires -again by deputyâ€”and you see bright sparks passing to tho earth. [Experiment shown.] Bear in mind that I am touching one wire only, not both, so that, although the Ruhmkoff coil is doubtless an old friend to you, I am using it to-night in a new way to illustrate my point.
Now let us consider why it is that when a current goes and returns by a wire completely insulated from the ground no shock is experienced when the current is perfectly steady, even if a bare part of the wire be touched; but if there are rapid variations in the current strength severe shocks are felt even if only one part of the wire be touched.
In each case the effect of touching the wire is to reduce the potential of the point touched to that of the earth, and to effect a corresponding alteration of tho potential along the whole line. This means the addition to a subtraction of a certain quantity of electricity from the wire. In the case of a perfectly steady current this operation only occurs once for once touching the bare wire, and unless the electrostatic capacity of the wire be very great, or the potential of the point touched immensely different from that of the earth, this single addition or substraction of electricity produces hardly any perceptible feeling. But in the case of a varying current the potential of the point would naturally be undergoing constant variations of potential, so that in order to keep the point touched at the same potential as that of the earth, constant additions and subtractions of electricity are necessary, and if these occur with sufficient rapidity a severe shock is produced, even although the body forms no part of the main circuit.
A single puff of air through this syren produces no audible sound, a rapid succession of even minute puffs a loud roar; so the single small electric discharge is not perceptible, but the rapid shower of Lilliputian arrows is terribly painful. This, indeed, is the same analogy Professor Perry employed in his lecture at the Society of Arts to prove the powerful action of rapidly recurrent effects in connection with our method of signalling to ships by means of sound waves sent through the water.
High potential, I have explained, is absolutely necessary for economy in electric transmission of power, and I hope I have also made it clear to you that, contrary to the received opinion, this high potential is not necessarily at all dangerous to life if the currents employed are not at all discontinuous. But the discontinuity of current we have to deal with is not merely the discontinuity that exists in the so-called reverse current machines. Even many dynamos, like the Brush, do not produce a really constant current, although generating a current of sufficient mean constancy to cause a perfectly steady deflection on one of the ordinary ammeters of Prof.Perry and myself, an instrument, be it remembered, so dead-beat that it records with perfect accuracy the change of current produced every time the joint in the driving-belt passes round the pulley of the dynamo, as well as the regular increase of current at each of the explosions of the gas in the cylinder of a gas engine.
The variations of current I am now referring to occur many hundreds of times a minutes, and although they cannot be detected by ordinary current meters, and although but little attention has been given to them, they are, in my opinion, a source of great danger when the dynamo produces a high electromotive force.
In any machine for the mechanical production of electricity a coil must, as you no doubt all know, be alternately rapidly brought up to a magnet and then withdrawn. Bringing up the coil produces a current in one direction in the coil, and withdrawing it a current in the opposite: and although by a simple piece of mechanism, called a commutator, the direction of the current in the external, or useful, portion of the circuit may be always kept the same, even although the direction of the current is alternately reversed in the coil no mechanism can make the coil produce a current at the moments of rest, when it changes its direction of motion. Wheatstone, in 1841, proposed to overcome this difficulty by combining two or more dynamos in series, like the two cylinders of a steam engine, in such a way that the current produced by the one was strongest when the current produced by the other was weakest; but Pacinotti, in 1860, showed us scientifically, and Gramme, in 1870, practically, how to combine a number of moving coils in one machine so as to give a current comparable in steadiness with that produced by a voltaic battery. The power of any dynamo depends roughly on the speed at which the coils move and on the strength of the magnetic field in which they move. Now, from the construction of the well-known Gramme machine, the strength of this magnetic field is necessarily weak when the machine is made of large size so that the greater power and efficiency which ought to come from enlarging the machine is partially compensated for by the relatively weaker magnetic field: indeed, even in a small Gramme machine the magnetic field is probably weaker than the magnetic field produced long ago in the old reverse-current machines of Holmes and Wilde. Hence more recent inventors, like Brush, have adopted a compromise between the perfect uniformity of strength of current obtained with the Gramme dynamo and the strong magnetic field in the old reverse-current machines. But in almost all the high electromotive force dynamos continuity of current has been somewhat sacrificed to strength of magnetic field. The plan, however, described in 1879 by Professor Perry and myself of oblique coiling of the armature, has enabled us, especially in our recently-constructed machines, to combine the continuity of current produced by a Gramme with the powerful magnetic field necessary for obtaining high electromotive force, and these results are in no way diminished when the machine is increased to the size necessary for utilising hundreds or thousands of horse-power and for obtaining that high efficiency which both theory and practice show is obtainable with the use of very large dynamos.
The next point to consider is how can we measure the amount of this dangerous discontinuity in the current produced by a dynamo without having to resort to the inconvenient process of killing off a certain number of people. We want a "discontinuity meter," and such an instrument I have here, employed by Professor Perry and myself for this purpose. It consists simply of an induction coil in which both the primary and secondary circuits are of thick wire. The one is put in the main path of the current, the other is attached to one of our "Spring Ammeters," which measures the mean square of the strength of the reverse currents produced in the outer secondary coil by the intermittencies of the main current.
Now, what are the results obtained when such an instrument is practically employed? Why, we find that when a current of ten amperes, produced by a three-light Brush dynamo, is flowing throngh the primary, there is a reverse current through the secondary which increases to about five amperes, when the secondary is nearly short-circuited, and which, be it remembered, is produced merely by the commonly ignored discontinuities in the current in the main current produced by the Brush machine. On tho other hand, when the same current, ten amperes, is sent through the primary by a Gramme machine running at about the same speed, no visible current, certainly not a quarter of an ampere, is produced in the secondary. Hence it is clear that the continuity of the Gramme current is immensely greater than the continuity of the Brush current.
A telephone may also be used as a discontinuity meter, the two terminals of which are in different experiments attached to two points close together in the main circuit of the dynamo. For quantitative experiments these two adjacent points in the main circuit should be selected, so that in all cases there is the same difference of potentials between them, and then the pitch of the note heard in the telephone will tell the number of variations per minute in the main current, and the loudness of the note the magnitude of such variation.
And I strongly advise any one who is procuring a high electromotive force dynamo to ascertain for himself, before purchasing, the magnitude of the discontinuity factor of the machine, and about which he will find no information, as a rule, given in any description of the machine.
The other inconvenience arising from the use of high potential and small current, viz., that as the motors and lamps must be put in series the accidental stoppage of the current passing through one stops the current passing through all, is easily remedied by the use of what are technically called "cut outs." A cut out, as you see, consists of an electro-magnet wound with comparatively fine wire, and which allows a small part of the total current to pass throngh it instead of through a particular motor or lamp. If, however, from no failure in the generator, no current can pass through the motor or lamp, then more passes through the cut out, the electro-magnet of which becomes stronger than before, attracts the armature which closes this side circuit, and so allows all the current to flow by the bye-pass thus formed.
We have seen, then, that for economic electric transmission of power high potential must be used; secondly, to avoid danger to workmen and others likely to touch the wires, both the going and return wires must be insulated, and also the current must not only be what is practically called a constant current, but must not be subject to the rapid variations in intensity such as are produced by many existing dynamos.
Next, in a satisfactory system of distribution of power, be it electric or otherwise, the supply should be exactly equal to the demand;a greater or less demand for power on the part of any consumer should not be accompanied by a less or greater supply to any other consumer of power. In the distribution of gas to a town this problem has never been completely solved. Owing partly to the mass possessed by the gas in the mains no automatic regulator has over yet been devised to use at the gas works so perfect that our burners do not flare at about nine o'clock in the evening, when the consumption of gas in the town is being rapidly diminished; and, indeed, at the present day a gas engine in a building even fed direct from the main will cause unpleasant pulsations in all the lights in the same building, also fed directly and separately from the main, so that the number of explosions in the gas engine can be counted all over the building. The pressure regulator at the gasworks cannot act quickly enough, so that Sugg and others have devised automatic regulators to attach to the burners themselves in the houses.
Constant pressure in the case of gas corresponds for electric lamps or motors in parallel, with constant difference of potentials between the leads and with constant current for lamps or motors in series.
As an illustration of the importance of these considerations you see this battery of three Grove's cells will light this incandescent lamp, or it will drive this electro-motor, but when I attempt to unite them in multiple arc together, the light of the lamp is, you observe, much diminished in intensity, due to the fact that the difference of potentials at the ends of the carbon filament is diminished when the motor is started. Again, I have those two lamps in series; I now allow the current to pass through a third lamp in the same series, and you observe that all the three are duller than either were before, due to the current being diminished. [Exp. shown.]
And, conversely, if from a sudden diminution in the consumption, arising from many of the lamps being suddenly turned off the difference of potentials between the main wires is much increased, all the remaining incandescent lamps are suddenly broken. Indeed, when the governing of the difference of potentials or of the currents was much less perfect than at present, it was no uncommon thing for hundreds of incandescent lamps, costing many pounds, to be suddenly broken.
Consequently, the problem of supplying either constant difference of potentials in the parallel system, or constant current in the series system, independently of the consumption of electric energy, and which might at first sight appear rather a dry and uninteresting problem, is in reality of great practical importance, since it must be solved before one train on an electric railway shall not suddenly start going faster because another has stopped, or before the electric lights in one train shall not become dim or break because the passengers in another train have turned theirs on or off. It is a problem, therefore, to which electrical engineers have devoted much attention during the past few years.
In my last lecture, I explained why small current combined with great electric pressure or high potential must necessarily be used for economy in electric transmission of power, and why, therefore, the series working of lamps and motors was better than the parallel system. I went pretty fully into the consideration of what was necessary to obtain safety, and we found experimentally that two conditions were essential: the first, that both the going and the return wires should be well insulated from one another as well as from the earth; the second, that the current should be absolutely constant in its strength, and not merely have the mean constancy obtainable with many of the so-called constant current dynamos.
The very important problem of supplying constant electric pressure to the mains, however many lamps or motors were being fed by them in parallel, was next considered, as well as the problem of supplying constant current to motors or lamps in series, however many lamps or motors were being worked in one circuit.
We ended by considering the great importance of driving quick moving tools like drills, fans, circular saws, &c, directly with electro-motors without intermediate gearing, because with such arrangement we saved first the loss of power that occurs in the shafting of a factory; secondly, the loss of power at the intermediate gearing employed to make the drill, or other quick moving tool, revolve much more rapidly than the shaft, and, lastly, we secured the great convenience of being able to bring the machine-driven tool to the work, instead of, as at present, the work to the tool. And there is one case where the driving of fans by electro-motors appears to me to be at the present moment of vital importance to us as citizens of London, I might almost say of national importance to us as Englishmen. For years past we have gone on breathing the foul air in the underground railway. Much grumbling has been the result, but hitherto no ventilation, and at the present moment we are contemplating, as a sort of final resource, the possibility of unsightly openings being made in the roadway of the Embankment to let out the obnoxious gases. Now why, I ask, do we not put fans at suitable intervals along the tunnel attached to quite small pipes for extracting the bad air? The fans could not, of course, be driven by steam-engines (there are steam engines enough in the tunnel already), nor could they easily be driven by shafting or belting, but an electro-motor attached directly to each fan would furnish the driving power without introducing any inconvenience whatever. No gearing would be necessary, a single insulated going and return wire would convey from motor to motor the electric power produced by a large dynamo outside the tunnel, and a steam engine might puff away in some back yard, out of everybody's way, and give us perfect ventilation on the underground railway.
And when on the subject of electro-motors, I explained that Professor Perry and myself had been led to construct electro-motors on a different principle from that usually adopted. In a dynamo machine there are two electro-magnets, and the machine acts as a producer of electricity because the work that is necessary to be done to move one of the magnets relatively to the other to overcome their attractions and repulsions is turned into electric energy. In practice one of these electro-magnets is usually kept at rest, while the other is in motion. In one of them the current always flows in the same direction, while in the latter it is constantly being reversed in direction. The former is called the "field " or inducing magnet, while the latter is the "armature." Now, it can be shown that in every dynamo-machine the magnetism produced by the current flowing round the armature must necessarily weaken the magnetism produced by the current flowing round the field-magnets, and hence must necessarily diminish the power of the dynamo.
Hence the best dynamos have been so designed that while the exciting electro-magnets form a very strong magnet the armature only produces a very weak one. This result has been arrived at partly by making the armature of a dynamo small, compared with the field electro-magnets, and partly by giving it a squat form; and because the field-magnet is large, it is, as a rule, kept stationary, while the armature revolves inside its poles.
I went on to explain to you that Professor Perry and myself had concluded from certain theoretical and practical considerations, that in an electro-motor, on the contrary, the strengths of the two magnetic fields produced respectively by the field-magnet and by the armature should be equal, since the one, so far from weakening the other, can be made to strengthen the other, and since the armature from its shape is necessarily rather a weak magnet, whereas the field, or exciting magnet, from its shape is a strong one, that we had reversed the usual condition of things, and made the armature large and the field-magnet small, and that from this we had been led to make the armature stationary and surrounding the field-magnet, instead of, as is usual, the field-magnet surrounding the armature.
Our small field-magnet, then, carries the brushes and revolves inside the stationary armature, the coils of which are joined to the stationary commutator, which, unlike ordinary commutators, we make flat to save both space and expense.
Wherever the brushes happen to be at any particular moment, there two opposite magnetic poles are produced on the armature. As the brushes run round and round so do these poles, and the brushes, which, be it remembered, are carried by the field magnet, are so set that the magnetic poles in the armature are always a little way in front of those in the field-magnet. The latter, therefore, are perpetually running after the former, but never catching them.
The law connecting the power of electro-motors with their size I dwelt on, and I explained that if the length, breadth and height of a motor be doubled, so that its weight and cost becomes eight times as great as before, its power is about 20 times as great as before, and its efficiency or the ratio of useful power given out to electric power put in is also much increased.
I ended my lecture by referring, but from want of time only very briefly, to the governing of motors. An ordinary ungoverned motor goes very much faster when running empty than when there is a load on, but in practice it would never do for a piece of wood or iron in a lathe to rush round when the chisel was taken off, since not only would there be a great waste of power, but on next applying the chisel it would be probably broken by the wood or iron when rotating with such an excessive velocity. Not only, then, must we arrange in the way I described, that the supply of electric power is proportional to the demand, but also that the motors consume this power in proportion to the work they have to do; in other words, the electro-motor must be governed so as always to run at the same speed, whatever amount of work it may happen to be doing. I referred to the oldest form of governor for motors, which I have called the "spasmodic governor," and which I will now show in action. [Experiment shown and full explanation given.] The great objection to it is that it either supplies full power when the motor is running too slowly, or no power when it is running too quickly, and, therefore, cannot produce a constant speed. Our "periodic governor," which never supplies full power or no power, but always an amount of power exactly in proportion to the demand, I very briefly referred to, and which I will now describe to you more in detail [Ayrton and Perry's "periodic governor" shown in action and fully explained]; and, lastly, I referred to the most advanced system of governing motors, which we had arrived at by winding the motor with two distinct circuits in such a way that the current passing through one of them magnetises the iron, causes the machine to act as a motor, and, consequently, is itself resisted, whereas the current passing through the other circuit tends to demagnetise the iron and stop the motion, and, consequently, is itself helped on; in fact, we have combined a motor and a dynamo in one machine, and so have dispensed altogether with anything of the nature of a mechanical governor. In the spasmodic, as well as in our periodic governor; the regulating action is produced by the supply of power being cut off when the motor is beginning to go too fast, but in our combined motor it is usefully employed to reproduce electric energy, which is added to the supply in the main circuit. In the one case, when less work has to be done by the motor, less electric power is given it, while with our combined motor the supply always remains the same, but when much work has to be done by the motor, nearly all the electric supply is turned into useful mechanical work; whereas, on the other hand, when little mechanical has to be done, a portion of the supply, after being turned into mechanical work, is re-converted into electric energy, and added to the supply going to other motors. The difference in principle is of this nature â€” the periodic governor corresponds with a man who, when he has not much work to do, does not eat much, lives economically; in fact, whereas our combined motor corresponds with a man who always eats the same amount every day, but when he has little work to do on his own account, immediately goes and helps somebody else who is hard pressed with work. [Experiments were then shown with the combined motor, and it was proved that the change of load which diminished the speed of the ungoverned motor from 1,560 revolutions per minute to 508, only reduced the speed in one of Ayrton and Perry's governed motors from 1,250 to 1,181 revolutions per minute.]
But we find that it is not even necessary to wind the motor with two distinct circuits in order to obtain a certain amount of governing, and this arises from the fact that our machine will act as a motor without any winding at all on the revolving field-magnets. Here is an example of such an unwound motor. I pass a current through it, and it at once commences to revolve, and, as you see, revolves rapidly. [Experiment shown.] This arises from the fact that the magnetism in the stationary armature induces magnetism in the iron of the field-magnets, and the brushes are so placed that the magnetic poles in the armature are always just in front of those in the iron, which latter are always running round and round after those in the former, but never catch them up.
In some of the early electro-motors pieces of soft iron moved in the neighbourhood of stationary electro-magnets, but in that case the continuous motion of the iron was maintained in a totally different way from that employed in our unwound motors, because in those older motors a piece of iron was pulled towards a stationary electro-magnet, opposite which it would have stopped, but that just as it was approaching the end of the electro-magnet the current was stopped and the iron went on by its own momentum; the pulls were therefore intermittent and not continuous, as in the case of our unwound motor.
Now, suppose we have one of our motors adjusted to run without any winding on the field-magnets, and imagine we wind the field-magnets in such a direction that the motor would tend to turn in the opposite direction when a current passed round the coil wound round the field-magnets, what would happen? Why, if the magnetism induced in the field-magnets by the current flowing round it were stronger than the opposite magnetism induced in the iron by the armature, we should have the very curious and novel result that the direction of the motor could be reversed by simply short-circuiting with a thick piece of wire, the current flowing round the field-magnets, and this result we have practically attained, as I will now show you. [Experiment shown.] Allow the current to flow through the armature and field-magnets, and the motor runs one way; let it only flow through the armature alone, and the motion is instantaneously reversed. Merely pressing down the key then reverses the motion. Hitherto the much more complicated arrangement of reversing the electric connections between the armature and field-magnet has always been thought necessary to reverse the direction of motion of a motor.
We have in the preceding considered that the magnetism produced by the coil on the field-magnets was greater than that produced by the induction of the armature itself, and so determined the direction of motion. But suppose that is not the case, then what will happen? Why, then the current passing through the coil on the field-magnets will be resisting the motion, and so will be helped on. But that, as we saw before, is exactly the condition necessary for governing. The armature then and the iron of the field-magnets acts as a motor, while the armature and the coil wound round it act as a dynamo, and if the resistance of the coil, which we make of fine wire, and as a shunt to the armature, is of proper resistance, and the required speed of the motor is the critical speed of the dynamo, the governing of our motor may be made fairly perfect with only one circuit on it.
Hitherto we have been dealing with motors especially intended always to run at one speed whatever amount of work they might be doing. We next come to motors for electric tramcars, tricycles, &c., designed to run at any desired speed forwards or backwards.
I have spoken about the set of the brushes relatively to the field-magnet, and the set, or "lead" as it is technically called, is of great practical importance, since on the amount of lead depends which way the motor revolves or whether it revolves at all. Although the fact that the direction of rotation could be reversed by a sufficient change in the lead was well known, the importance of varying the lead in motors for different speeds appears to be little attended to, since it is impossible as a rule in motors, from their construction, to make small changes in the lead. In our motors, on the other hand, the flat brush-holder can be revolved forwards or backwards by hand through any angle, so that any lead forwards or backwards is obtainable, and in our larger motors, of which I have a sample here, the lead can be made anything we like simply by moving a handle, such as a locomotive engine-driver is accustomed to use for acting on the link-motion of his engine. In fact, by moving the handle forwards or backwards any speed, in either direction, is obtainable. We can, even when the motor is running, shift the brushes relatively to the field-magnet together with which they are rotating, and consequently with only one pair of brushes to give any lead forwards or backwards we desire. In other cases we alter the lead by means of a wheel and screw, and so get very easily a very accurate adjustment. A table of speeds can be engraved, showing the position the handle should be in for 500 revolutions a minute, or 700 revolutions a minute, &c.
On the last occasion I showed you photographs of all the principal electric railways, and we saw that in some cases the current went by an auxiliary insulated rail passed through the motor under the train, and came back by the rails. In other cases both the going and return wires were of the nature of stout overhead telegraph wires, and a running connection between the motor on the tramcar and these wires was kept up by two small jockeys that were pulled along the telegraph wires by wires attached to the tramcar. In a third system, the going and return conductors were simply ordinary rails on which the train runs, the insulation produced by the rails merely resting on wooden sleepers having been found sufficient for a length of one or two miles. As you will see from the photographs that I now project on the screen, this last and simplest plan has been adopted for the Portrush Railway, near the Giant's Causeway, in Ireland. This electric railway, Mr. Traill (the chairman) has been so good as to write to me, "runs on a footpath, or trampath, by the side of the road, which has been widened for the purpose from 24ft. to 27ft. The path is 7ft. wide, leaving 20ft. for the carriage-way. "We first tried," he says, "sending the current by one rail, returning by the other rail, after passing across by the car, and for this purpose had laid the rails in asphalte with thick felt underneath, but we found that the leakage, especially in wet weather, was too great to justify us in continuing that kind of insulation for six miles." Now, this is exactly the result I predicted in my lectures on electric railways delivered last year, and in order to make the difficulty quite clear to you, I will show you the experiment I then showed to prove this. A portion of the going wire conveying the current working the motor is driving the circular saw, as you observe, wrapped round this bit of wood, and a portion of the return wire round this other piece of wood. As the two pieces of wood are dry there is very little leakage from the wire on the one to the wire on the other, although the wire wrapped round each is bare. If, however, without allowing these pieces of wood to touch one another, I immerse the ends of them in this vessel of water I shall produce the same effect as if a shower of rain fell at one end of the electric railway. Now, observe what happens, the motor immediately begins to go more slowly. Let me immerse them further, which will correspond with the rain becoming more general over the line, and the motor goes still more slowly, and now the motor stops altogether, so much of the current, in fact, leaking from the going to the return wire through the water that sufficient does not pass through the distant electro-motor to keep up its motion.
How, then, has this difficulty been overcome at the Giant's Causeway? Mr. Traill's letter tells us. He says :â€”" We have therefore placed a third rail of iron on wooden props close into the fence, so that a brush striking out from the car on a steel bar runs along the top of this third rail, which is about 15 inches from the ground." He goes on to say that this third rail is embedded in tar on the tops of the posts, and that when, in addition, pieces of the new insulating material (" insulite") are introduced between the rail and the wooden posts they expect the insulation will be perfect. He adds :â€”" At present we can run for over two miles on the road up and down hill gradients as great as 1 in 35. We took six tons up that incline the other day. We are about to erect turbines at the waterfall above Bushmills, which will give us from 60 to 100 horse-power, and will generate the current there about half a mile from our main line at Bushmills. We have powers by our Act of Parliament to construct a railway from Bushmills to Dervock (inland about seven miles) almost at right angles to the line of tramway on the sea coast. As the waterfall is near the junction of railway and tramway we intend to use it for the railway as well, as the works will do for both. We are limited by an Act to 10 miles an hour on the tramway, but will have no difficulty in going to 20 or 26 miles an hour on the railway, which has also the advantage of being level the entire way."
But although they have succeeded at Portrush in overcoming the difficulty of leakage by using this third insulated rail, resting on posts 15in. above the ground, such a system presents, you will easily see, great inconveniences for ordinary railways, since at every crossing there would be this raised rail in the way of the trains, and an even greater objection is the risk that it introduces, that any one can, from carelessness or malice, by resting, say, a crowbar against this raised insulated rail, send all the electricity to the earth and so cut off all electric power from all the trains either in front or behind.
On the other hand, the plan of using stout telegraph wires, with running jockeys to keep up continuous electric connection between these wires and the moving train, would present great mechanical difficulties if the trains were going at 60 miles an hour.
What, then, do we propose? Professor Perry's and my plan is this: we electrically subdivide the rubbed rail into a number of sections all fairly, but by no means perfectly, insulated from the ground. We do not apply our electricity directly to this rubbed rail, but instead, to a well-insulated conductor, which may be buried underground or may be insulated by resting like a telegraph line on insulators or posts, and we arrange that whenever a train enters on to any section, it automatically makes an electric connection between that section of the rubbed rail and the well-insulated conductor which supplies the power, and at the same time automatically cuts off the electric power from the section the train has just left, and in this way we confine the leakage of electricity to that section on which the train is at any particular time, and so reduce the leakage and waste of power on the longest electric railway to less than it is at present on the shortest line constructed on the Berlin plan.
But not only does a train on entering any section automatically turn on the electric power to that section, and cut it off from the one just left, but it accomplishes something more; it blocks absolutely the section just left. Let us take an example :â€”Suppose a train is passing from section A to section B, then not only is the power automatically turned on to section B and cut off from section A, but no following train entering on section A can receive power and move on as long as the train in front is on section B. It is not until the preceding train has passed into section C that the following train can proceed along section A. And, in addition, whenever a train is approaching too near a train in front, and in consequence finds itself deprived of driving power, it is automatically, by a special plan we have contrived automatically, powerfully braked.
Whenever, then, a trainâ€”it may be even a runaway engineâ€” enters on a blocked section, not only is all motive power withdrawn from it, but it is in addition automatically powerfully braked quite independent of the action of signalman, guard, or engine-driver, even if the latter two be present, which, bear in mind, is not at all necessary with our electric railway. No fog nor colour blindness, nor different codes of signals on different lines, nor mistakes arising from the exhausted, nervous condition of overworked signalmen can with our system produce a collision. The English system of blocking means merely giving an order to stop, but whether this is understood or intelligently carried out is only settled by the happening or non-happening of a subsequent collision. Our absolute automatic block acts as if the steam were automatically shut off, and the brake put on whenever the train is running into danger; nay, it does more than this, it acts as if the fires were put out, and all the coal taken away, since it is quite out of the power of the engine-driver, if there be one, to re-start the train until the one in front is at a safe distance ahead.
At present much household work is done by hand, simply because there are no easily-worked machines for doing it. The old knife-board has given way to the rotary knife-cleaner, but even that requires a certain amount of grinding to give the knives a polish, so that for large establishments a knife-cleaner boy is still necessary. The blacking of boots, the blacking of grates, the cleaning of doorsteps, &c, are all done in a most laborious way by hand. (I might almost say the operation seems not to be confined to hand, judging from the generally smutty appearance of Sarah Anne after the process.) Now, there can be no doubt that very shortly electricity will be supplied, as gas is now supplied, to houses for lighting purposes, and when this has been accomplished, the same wires that convey the electricity for lighting during the night will be employed in the day to convey the power to work electric-motors to turn rotary knife-cleaners, to turn a wheel for the blacking of boots, and a small motor carrying a brush like the one in my hand will simply be passed by the servant all over the grate for the purpose of giving it a good black polish. The black-lead brush will then be taken off, and replaced by the blacking brush for the boots, and later on in the day a rotary flannel will officiate for the door-steps.
The high price of land in towns necessitates gas works being in the suburbs, so in the future the electric works must be in the country, and probably farther away from the towns than even gas works are at the present time, because not only will great economy be so attainable, but the town will be freed from the smoke of the engines to drive the dynamo machines or producers of electric current, and by that time we shall have begun to realise that a smoky atmosphere is a poisonous atmosphere; further, if our dynamos are far away in the country advantage can also be taken of natural sources of water power.
The transmission, then, of power by electricity is not only a problem which must be solved, but is one which calls for immediate solution.
For the economic electric transport of power we must produce, as I have explained, a great difference of potentials by a dynamo at the place where the power is obtainableâ€”in the suburbs where, for example, is the steam engine or water wheelâ€”and employ some mechanism which can be worked by this great difference of potentials at the place where the power is to be utilisedâ€”that is, in the town. There is no difficulty in arranging a dynamo wound with fine wire, or a set of dynamos in series with one another, to produce any difference of potentials required; but there is a far greater difficulty in using this great difference of potentials at the town end of the line. If lamps and motors of the present construction are used they would have to be put in series, but in that case if the current is stopped passing through one it will be stopped passing through all. This difficulty can to a great extent be avoided by using "cut-outs," as I described to you in my last lecture. At the same time nobody would be perfectly satisfied that his supply of electric power should depend on the good working of all the electric apparatus in the same street, much less that it should depend on the good working of all the electric apparatus in the same town. We are content that our supply of gas or water should depend on the good working of the gas or water works, but we could not tolerate the possibility of Betsy Jane, in Moorgate-street, from some carelessness or other, being able to cut off our supply at the London Institution. That Jack Frost should stop our supply is bad enough, but Betsy Janeâ€” no!
What, then, are we to do? High difference of potential we must use, and we cannot work the lamps and motors in parallel unless they have a resistance of tens of thousands of ohms, in fact, are of a totally different description from the lamps and motors of the present day. The solution was first suggested by Sir William Thomson in 1881, and, I am happy to say, will very shortly be carried out in Paris by Professor Perry and myself, under the auspices of the French Electrical Storage Company. It is to use the small current and great difference of potentials to charge a very large number of accumulators, in series, in the town, and to discharge them in sets, as shown in the figure, each set supplying the energy for one street, or, possibly, for one district. Of course, the going and return wires between the dynamo and accumulators must be separately insulated, and put so far apart that there is no fear of a man or an animal accidentally touching both. If, in addition, it is feared that although this precaution is taken still somebody may touch a leading wire, say A, coming from somewhere near one end of the accumulators, while another person may accidently touch a leading wire, B, coming from somewhere near the other end of the accumulators, and so both receive a fatal shock, this may be also entirely avoided by electrically disconnecting the various sets of accumulators from one another, and from the main charging wires coming from the dynamo before they are connected respectively to the local leads in the streets for being discharged.
Suppose a thousand accumulators were thus employed, and a charging current of 33 amperes were used. To balance the electromotive force of the accumulators about 2,100 volts would be required. If the resistance of each cell were 0.005 of an ohm during charging, then 165 volts difference of potentials would be required to send a current of 33 amperes through this resistance, or a total difference of potentials of 2,265 volts would have to be maintained at the terminals of the accumulators. Hence about 100 horse-power would be put into them.