522 



NA TURE 



[October 



a potential difference in the transmission higher than that 

 maintained between the terminals of a lamp. 



Two wrong notions misled people in those days — the 

 one, that the maximum efficiency of a perfect electromotor 

 could be only 50 per cent. ; the other, quoting the remarks 

 of Sir W. Siemens in the discussion of the paper read 

 by Messrs. Higgs and Brittle at the Institution of Civil 

 Engineers somewhat later in the same year 1878, "In 

 order to get the best effect out of a dynamo-electric 

 machine there should be an external resistance not 

 exceeding the resistance of the wire in the machine. 

 Hitherto it had been found not economical to increase 

 the resistance in the machine to more than one ohm ; 

 otherwise there was a loss of current through the heating 

 of the coil. If, therefore, there was a machine with one 

 ohm resistance, there ought to be a conductor 

 transmitting the power either to the light or the electro- 

 magnetic engine not exceeding one ohm." He then goes 

 on to consider that as the conductor is lengthened its 

 cross-section must be increased in proportion to keep the 

 resistance constant at one ohm ; and he arrives at a result 

 quite new at the time, viz. that if the number of dynamos 

 in parallel were increased in proportion to the length and 

 cross-section of the line, "it was no dearer to transmit elec- 

 tromotive force to the greater than to the smaller distance." 



Sir William Thomson grasps at once the novelty and im- 

 portance of this idea, and renders it even more important 

 by proposing to put all the dynamos in series at one end 

 of the line, and all the lamps in series at the other. But 

 it would still appear that even 40 per cent, efficiency for 

 transmision over a considerable distance could only 

 be attained when " there were a sufficient number of 

 lamps " to make it necessary to use ittany dynamos in 

 parallel in accordance with Siemens's proposal, or, many 

 dynamos in series in accordance with Thomson's 

 modification of Siemens's proposal. 



In 1879, the electric transmission of power was still 

 such a terra iftcogttita that the largest firm of electrical 

 engineers in Europe could not be induced to tender for 

 transmitting power over ten miles in India. 



At the British Association lecture in the autumn of 1879, 

 Prof. Ayrton exposed the fallacy of assuming that 50 per 

 cent, was the maximum efficiency theoretically obtainable 

 with an electromotor. He further proposed that, instead 

 of employing many dynamos at one end of the line and 

 many lamps at the other, there should be used a single 

 dynamo and a single motor, with much wire on each ; that 

 the high potential of the line necessary for economical 

 transmission of power should be maintained by running 

 both dynamo and motor much faster than hitherto ; and 

 that both dynamo and motor should be separately excited. 

 Although not wholly free from the prevailing idea of that 

 day — that electric transmission of power over long distances 

 would only be commercially possible when a very large 

 amount of power had to be transmitted — he says, after 

 discussing the subject, " So now we may 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, to be able to be diminished with our best 

 existing dynamo-electric machines to about 30 per cent." 



This was perhaps the first time that it had been even 

 suggested that the efficiency in electric transmission of 

 power could be more than 50 per cent. 



Further, the lecturer proposed to use in all cases this 

 high E.M.F. motor, whether the received power were 

 required for motive purposes, for light, or for electro- 

 plating ; and, as experimentally shown in the lecture, to 

 generate the current locally in the two latter cases by 

 using the motor to drive a suitable dynamo, thus giving 

 the first illustration of the employment of an electric 

 transformer in the actual transmission of power to a 

 distance. 



NO. 1 144, VOL. 44] 



Two years later, viz. in 1881, the old mistaken notion, 

 that it was only 50 per cent, of the power given to a 

 dynamo that could be returned by the motor, was again 

 propounded during a discussion at the Society of Arts ; 

 and the Chairman, Sir W. Siemens, when correcting the 

 speaker's error, added, " Experiments of undoubted ac- 

 curacy had shown that you could obtain 60 or 70 per 

 cent." 



In this year two very important propositions were put 

 forward — the one, by Sir W. Thomson, at the semi- 

 centenary meeting of the British Association, that, in the 

 electric transmission of power, the small current of high 

 potential difference should be employed at the receiving 

 end of the line to charge a large number of accumulators 

 in series, the accumulators being subsequently discharged 

 in parallel for supplying light or power to a town ; the 

 other, by MM. Deprez and Carpentier, to use one alter- 

 nate current transformer at the sending end to raise the 

 electric pressure, and another transformer at the receiving 

 end to lower it down again, the arrangement being sym- 

 bolically shown in Fig. i. 



Fig. 1. — Deprez and Carpentier's Plan of Double Transfonnation 



The great advantage of this combination is, that the 

 pressure along the line may be very high, and the line 

 therefore composed of only thin wire, whereas the pres- 

 sure between the leads from the generating dynamo at 

 the transmitting end, as well as the pressure between the 

 lamp mains at the receiving end of the line, may be as 

 low as if the dynamo and lamps were close together. 



In the experiments, however, made in the following 

 year, 1882, to transmit power from Miesbach to Munich, 

 along thirty-five miles of iron telegraph wire o'i8 inch in 

 diameter, the current going by one wire and returning by 

 another, M. Deprez did not employ his double transform- 

 ing arrangement described above, probably because alter- 

 nate current motors were then quite untried practically. 

 But, instead, he used a direct current dynamo generating 

 a potential difference of some 1500 volts, the current from 

 which set in motion a direct current motor, wound to 

 stand a similar high pressure, placed at the other end of 

 the telegraph line. 



The experiments were attended with various break- 

 downs of the dynamo, which was probably constructed 

 on the usual string-and-glue fashion of those days ; and 

 finally, after repairs had been effected, the power given 

 out by the motor at Munich was only a fraction of 

 I horse, with a commercial efficiency of about one-third. 



It was, therefore, decided to repeat the experiments 

 the next year, 1883, with machines constructed more 

 solidly, and for the convenience of the jury the dynamo 

 and motor were placed close together in the workshops of 

 the Northern Railway near Paris, one terminal of each 

 being connected by a short wire, and the other terminals 

 by a telegraph wire 0-157 inch thick going from Paris to 

 Bourget and back again, a distance of 18,133 yards. The 

 power used in driving the dynamo was towards the end 

 of this second set of experiments about 10^ horse, and 

 the power given out by the motor about 3A horse, the 



