Oct. 9. 1879] 



NATURE 



569 



pound of ziuc ; but we must consider what fraction of the heat 

 thus produced is converted into useful work in a heat and in an 

 electric engine respectively. 



"As already mentioned, our most perfect steam-engines can be 

 made to produce one-horse power with the consumption of 2 lbs. 

 of coal per hour. Now, the burnin;j of 2 lbs. of coal will pro- 

 duce enough energy to raise 18,528,000 pounds one foot, or will 

 produce 18,528,000 foot pounds of work. Now, one-horse 

 power is equivalent to 1,980,000 foot pounds of work per hour ; 

 therefore, as regards the total energy in coal, even our best 

 steam-engines only utilise ^ of it, and waste \. But, as already 

 mentioned, even a perfect engine cannot, with the ordinary tem- 

 perature available, utilise the whole of the heat of the futl. In 

 fact, theory tells us that the efficiency of a perfect heat engine, 

 or the ratio of the work done to the maximum work obtainable 

 from the consumption of the fuel, is equal to the ratio of the number 

 of degrees of temperature through w hich the steam is cooled in 

 doing work to the highest temperature of our steam, when we 

 take as our zero of temperature a point 460° below the ordinary 

 zero of the Fahrenheit scale. 



" Now, in our best steam-engines, the steam, when it begins to 

 push the piston by expanding, has a temperature of about 300" F. , 

 and at the end of the stroke a temperatiu-e of 100° F., so that 

 the efficiency of a perfect ideal engine working betn een these 

 temperatures is only about |, not so very much greater than that 

 of best practical engines. 



" No great advance can be made, then, in a heat engine, except 

 by making the temperature of the working substance, steam, gas, 

 or whatever it may be, much higher. If, for example, we could 

 raise the temperature of the working substance as high as, say, 

 3,000° F., the temperature of combustion, and could make it 

 leave the engine without artificial cooling at the ordinary tem- 

 perature of the air, which is, say, 60° F., then a perfect heat 

 engine, under these conditions, would only waste about | of the 

 total energy ; consequently, assuming that we could, at these 

 high temperatures, make a practical engine as good relatively to 

 an ideal perfect engine as we can at lower temperatures, then a 

 practical engine would only waste /j of the total energy, or 

 would have an efficiency of about 0'84. 



" J3ut, with our present knowledge, to work with steam or gas 

 at a temperature of 3,000° F. is almost as ideal as an engine 

 with no friction and with no loss of heat by conduction and 

 radiation. We are, therefore, led to the conviction that as it is 

 solely by working with steam at very high temperatures that the 

 efticiency of steam-engines can be seriously increased, it may be 

 well to consider whether it is not possible to economically replace 

 the steam engine with some other form of motor." 



It was then proved theoretically and experimentally that when- 

 ever an electro-motor is being worked by an electric current it is 

 acting as a magneto-electric machine and producing a reverse 

 current tending to stop the motion. 



The lecturer then explained that, when an electro motor is 

 worked by a given galvanic battery, calculations lead us to the 

 result that if we wish to produce the work most economically we 

 must, by diminishing the load on the motor, allow its speed to 

 increase until the reverse current it produces is only a little 

 smaller than that sent by the battery ; in fact, until the ciurent 

 circulating through the arrangement is very small, in which case 

 the efficiency of the engine, or the ratio of the work it produces 

 in a given time to the maximum work it could produce from the 

 same consumption of material is nearly unity. If, on the other 

 hand, we desire a given battery to cause the motor to do work 

 viosl quietly, indeiJcndently of the consumption of materi.il, then 

 calculation tells us that we ought to put such a load on the motor 

 that its sjieed will send a reverse current equal to something like 

 a half of the strength of the current the battery could send 

 through the motor when at rest. In this case the efficiency is 

 about J, or half the energy is wasted in heat. 



He impressed upon the audience that the difference between 

 these two considerations of maximum values ought carefully to 

 be borne in mind, especially as it was usually the second — or 

 how to obtain work most quickly — that had generally been taken 

 into account, whereas it was the other one — or how to transmit 

 work most economically, that woiJd specially engage their atten- 

 tion during the lecture. 



And in connection with the latter maximum value-he said, " Let 

 us consider that we work our motor in the most efficient way — 

 that is very fast with a small load, and let us suppose as an 

 extreme case th.it by so doing the efficiency is so little short of 

 imity that we may regard it as one ; then since an electro-motor 



worked by a battery in which zinc is burnt is 150 times as costly to 

 maintain as a steam engine for equal efficiencies, the best electro- 

 motor worked by such a battery will be thirty-three times as dear 

 as our best steam engines having an efticiency of J. We may, 

 therefore, throw on one side at once all idea of electro-motors 

 worked by ordinary batteries, even although the electro-motors be 

 perfect. Noh', this result is most important, since it shows not 

 that an electro-motor as a machine is inefficient, but it tells us 

 that attempting to drive it with a galvanic battery is the hope- 

 lessly inefficient part of the arrangement. 



" But if we turn to the question of using electro -motors for the 

 transference of power, then there is no difficulty about burning 

 ziuc, and the high efficiency of such motors is all important. 



" For in the case of natural sources of power, such as waterfalls, 

 we have merely to consider what amount of energy will be pro- 

 duced at the distant factory ; will it be sufficient to repay the 

 expense of putting up wires from the source to the factory, 

 together with the cost of the two dynamo-electric machines, or 

 will it be cheaper to put up and use a small steam engine having 

 probably an efficiency of only y'j ? 



" When the distance between the source and the motor is con- 

 siderable, the cost of putting up the leading wires becomes 

 important, and the question therefore arises, can two or more 

 people use the same leading wires without increasing the thick- 

 ness, or must the thickness of the wire be so much increased as 

 to make the construction of two sets of leads as economical ? " 



Prof. Ayrton explained that he attached great importance to 

 this question because the answer to it would decide whether the 

 electric transmission of power was a mere dreamer's fancy, or 

 was likely to have a real commercial future. 



A detailed examination was then made of the laws governing 

 the transmission of energy by water power, and as a result of the 

 fact that the energy of a flow of w ater depends on the quantity and 

 on its head it was shown that, as far as the waste of power by 

 friction of the 'water in the pipes was concerned, a gi-eat pressure 

 in the reservoir sending a small current to turbines in a towa also 

 working at great pressure was an extremely economical mode of 

 transporting power, but that if we look into account the inefficiency 

 of existing engines for producing a great pressure of water at the 

 reservoir, combined with the great waste of power arising from 

 even small leakages that were certain to be caused by the great 

 water pressure, it followed that the system was an impracticable 

 one. 



An examination was then made of the laws governing the 

 electric transport of energy, and the lectiu-er arrived at this 

 result : ~ 



"Just as we concluded in the case of the water, that the most 

 efficient method to employ in order to transfer the energy, was 

 great pressure in the reservoir, combined with turbines in the 

 town working at a high pressiu'e, so now we conclude that the 

 most efficient way to transfer energy electrically is to use a 

 generator producing a high electromotive force, and a motor 

 producing a return high electromotive force ; and by so doing 

 the waste of power in the transmission ought, I consider, be able 

 to be diminished with our best existing dynamo -electric machines 

 to about 30 per cent. ; for, as exiDcriment shows the efficiency 

 of our best existing dynamo-machines to be o'86 (that is 86 

 ]'>er cent, of the power spent in revolving the bobbins is repro- 

 duced as energy of electric current) ; therefore, if two similar 

 dynamo-electric machines be coupled up to transmit power, and 

 if they are worked most economically in the general way I have 

 already explained, and with the details of arrangement that I 

 will enter into later on, instead of being worked so that the 

 motor gives out power most rapidly, I have reason for expecting 

 that the combined efficiency of the arrangement can be made to 

 closely approach the square of o'86, and not merely one-half, as 

 commonly supposed. 



"But while_the two solutions of the problem are thus identical, 

 there is this most important dilTerence : increasmg the jjressure 

 of the water means an uneconomical task, while increasing the 

 electromotive force set up by a dynamo-electric machine, or an 

 electro-motor, means merely running it faster, or running it at 

 the same speed and putting more wire on the rotatory portion. 



"And again, assuming that the mean electromotive force be- 

 tween the w ire and the earth be as much as one hundred times the 

 electromotive force producing the current, namely, the difference 

 between the electromotive forces of the generator and of the 

 motor, then with the ordinary insulation of the best land telegraph 

 lines, less than one per cent, of the energy transmitted ten miles 

 would be lost by leakage. 



