4i6 



NATURE 



[August 30, 19CO 



He referred to the growing use of high pressure alter- 

 nating currents for transmitting power to tramways, 

 and performing a double transformation for supplying 

 the low pressure direct current for driving the electro- 

 motors on the cars ; and he considered that this un- 

 necessary complication arose from the tramway motor 

 having been developed as a direct current motor, and 

 from the difficulty that would now be experienced in 

 replacing the many tens of thousands of direct current 

 tramway motors with an alternating current type. In 

 the case of new tramways and railways, at any rate, 

 he looked forward to the time when the alternating 

 current would alone be employed, but he admitted that 

 the electric simplification would be accompanied with 

 greater risk of shock and danger to life. 



As to the interference that might be caused by electric 

 tramways to magnetic observatories, he thought that, in 

 view of the far greater commercial importance of the 

 tramway, the magnetic observatory would have to give 

 way, and remove its apparatus to a place where electric 

 tramways were not required by the public. 



Prof Ayrton expressed the view that, since no doubt 

 existed as to the considerable damage that electrical under- 

 takings had caused to underground pipes, telephones, 

 submarine cables and magnetic observatories, the question 

 arose whether an endeavour was to be made to prevent the 

 attack or to strengthen the defence. In the case of tele- 

 phone circuits the Joint Committee of the two Houses of 

 Parliament in England had decided that since — wholly 

 apart from the advent of electric tramways — the Tele- 

 phone Companies had realised that, in order to prevent 

 interference between the telephone lines themselves, 

 as well as to prevent the disturbance caused by neigh- 

 bouring telegraph lines, it was necessary to abandon the 

 earth-return and employ a metallic return, and since 

 such a metallic return would shield the telephone circuit 

 from disturbance that might otherwise be caused by 

 electric tramways, there was no necessity to debar the 

 tramway from employing the earth. 



But as regards the electrolytic destruction of gas and 

 water pipes the matter was quite different, and, therefore, 

 the Board of Trade had imposed a regulation forbidding 

 the difference of potential between any part of the rail 

 and the terminal of the dynamo being allowed to exceed 

 7 volts. Prof. Ayrton pointed out,: however, that this 

 limit was too high even to prevent electrolysis, and cer- 

 tainly would not prevent the mutilation of messages 

 received through a submarine cable which was landed in 

 the neighbourhood of an electric tramway, as instanced 

 at the Cape of Good Hope. 



He questioned whether the security anticipated by Mr. 

 Ferranti and others that would follow from a general 

 substitution of alternating for direct current would be 

 nearly as great as was imagined, and he referred to the 

 experiments which he had published some years ago on 

 the comparatively rapid production of separated hydrogen 

 and oxygen that could be obtained in an ordinary 

 sulphuric acid voltameter, through which an ordinary 

 alternating current was passing. The specimen of a pipe 

 corroded with an alternating- current of one ampere 

 passing for six weeks lying on the table, and which had 

 been sent to the meeting by Mr. Trotter, of the Board of 

 Trade, was an important illustration of the electrolytic 

 action that could be produced with the commercial 

 alternating current supplied by the London Electric 

 Supply Company. 



A magnetic observatory was in a more serious position 

 still, since, as the undisturbed magnetism of the earth 

 had to be measured, no system of defence could be 

 utilised, and nothing short of the absence of attack could 

 be satisfactory. He was glad, therefore, to say that the 

 Electric Tramway Companies in London, thanks to the 

 action of the Board of Trade in appointing a joint com- 



NO. 1609, VOL. 62] 



mittee to represent the commercial and the tramwray 

 interests, and thanks to the experiments and the negotia- 

 tions carried out by this committee during the past eight 

 months, had not regarded the preservation of magnetic 

 records from the drastic point of view advocated by Dr. 

 Kennelly. In fact, the president of their Institution, 

 Prof. Perry, in co-operation with Prof. Riicker, had suc- 

 ceeded in inducing the London Tramway Companies to 

 propose a scheme in which, first, all the lines within a 

 radius of two miles round the Kew Observatory should 

 be divided up into absolutely distinct one mile sections ; 

 secondly, that the current should be led to the trolley 

 wire and withdrawn from the rails at the 7niddle of each 

 of these sections ; and, thirdly, the difference of potentials 

 between the rails and the earth withm this two miles 

 radius should never be allowed to exceed one-fifth of a 

 volt. And with these conditions, calculation showed 

 that, although the protection afforded would not, of course, 

 be as good as that obtained with a wholly insulated 

 system, it would be probably sufficiently great to prevent 

 any appreciable interference being caused with the 

 magnetic observations regularly taken at the Kew^ 

 Observatory. 



M. Corda thought the adoption of the alternate or the 

 direct current was mainly a matter of cost, and since the 

 Fire Insurance Companies allowed the maximum pressure 

 to be used with the alternating current to be only half as 

 great as with the direct current, he considered that as 

 long as that regulation lasted the direct current must 

 gain the day. 



Prof. Crocker said that the interference produced by 

 an electric circuit on another undertaking might be 

 divided into that produced by induction and that pro- 

 duced by leakage. The disturbance of the apparatus in- 

 a magnet observatory was due to both causes, but, as 

 there were so few magnetic observatories in the world, 

 that particular disturbance might be dismissed fron^ 

 consideration. With alternating currents the disturbance 

 produced by induction was the more serious because 

 this induction set up currents in other wires, and it was, 

 therefore, very difficult to avoid. With direct currents 

 the leakage disturbance was the more serious, but it was 

 possible to prevent this. Some time ago he had had 

 occasion to test the insulation of the whole of the New- 

 York electric lighting system, which was split up into 

 sections for this purpose, and he found that the current 

 which leaked to earth did not exceed one per cent, of the 

 current that was supplied to the houses, whereas with the 

 gas system in New York from lo to 20 per cent, of the gas 

 was lost by leakage. Consequently, since very high 

 insulation could be obtained with the type of under- 

 ground cables that were employed with high pressure 

 work, it followed that the leakage on the low pressure 

 electric light system employed in New York could be 

 reduced to a still lower value than one per cent. Further, 

 that if it were possible in London to reduce the potential 

 difference between the rails and the earth to only half a 

 volt, he should imagine that electrolysis might be 

 avoided even with the ordinary trolley wire tramway. 

 He was, therefore, in favour of employing the direct 

 current for the purpose of avoiding interference with 

 other interests. 



But he considered that the considerations of economy 

 and efficiency were more important than those regarding 

 interference, and, while the three-phase and the direct 

 current motors of the same power had the same effi- 

 ciency from half up to full load, the direct current motor 

 was the more efficient for small loads. Further, while 

 for constant speed the regulation with both types of 

 motor was about the same, the direct current motor 

 had a distinct advantage in regulation when the speed 

 was variable. On the whole, therefore, he was in favour 

 of the use of a direct current system of electric supply. 



