On the Magnetic Field produced by Electric Tramways. 423 



of the compressor, motor, and Hampson air-liquefier, which 

 together cost about £200, is comparatively inexpensive. The 

 gasometer cost £15, the material used in making the liquefier 

 amounted to about £5, and possibly £30 was spent in the 

 experiments in addition to the sum named. Each time liquid 

 hydrogen is made 5 kilos o£ solid carbonic acid and 8 litres 

 of liquid air are used ; this involves a further cost of about 

 £1. These figures indicate that the cost of liquid hydrogen 

 is not excessive. 



I am much indebted to Mr. Holding, lecture-assistant in 

 the department, for assistance in constructing the liquefier 

 and in carrying out the experiments. I also wish to express 

 my most cordial thanks to Dr. W. Hampson for his valuable 

 advice and for the assistance which he has so willingly 

 rendered. 



University College, London. 



XXXVIII. On the Magnetic Field produced by Electric 

 Tramways. By A. W. Rttckek, F.R.8* 



THE following calculations were made during the inquiry 

 which has recently taken place on the Magnetic Field 

 produced by Electric Tramways. In the course of the dis- 

 cussion it became evident that the gentlemen who represented 

 the Tramway Companies had arrived at similar results ; and 

 Mr. Parry has published in the i Electrician ' for Aug. 10, 

 1900, a full account of this part of the theory. I had worked 

 out both the "source and sink" and the " Fourier- Bar " 

 theories, given below, before I was aware of the fact. The 

 former is perhaps as accurate as the assumption that the 

 earth is homogeneous will allow ; and it was a priori im- 

 probable that the Fourier-Bar theory would represent the 

 facts near the terminals of the line. The following results 

 confirm this view ; but this part of the subject was developed 

 in consequence of a statement by the Engineers that the 

 "Fourier-Bar" theory agreed with the results of experiments 

 conducted by themselves, and which are not further referred 

 to in this paper. 



There is no difficulty in the calculations ; and my onlv 

 object in publishing some account of them is to draw atten- 

 tion to the fact, which is not, I think, generally recognized, 

 that the leakage currents on a homogeneous earth affect 

 directly only the horizontal force, while the vertical dis- 

 turbing force is due only to the difference of the effects of 

 the currents in the trolley-wires and rails or other horizontal 



* Communicated by the Physical Society : read Dec. 14, 1900. 



