1894.] On obtaining a Unidirectional Current to Earth. 99 



surrounding the vessel by a powerful magnetising helix," also ob- 

 tained a negative result, perhaps for the same reason. 



Knott's* experiments were made with hollow iron tubes, 45' 7 cm. 

 in length, 3'84 cm. in external diameter, and of different bores, 

 ranging from 0'7 cm. to 3*19 cm. " Each tube was closed below, and 

 into the upper end a nut screwed tightly, through a perforation in 

 which issued a fine capillary glass tube. The nut was adjusted under 

 water, so that the whole of the interior space of the metal tube was 

 filled with liquid, and also part of the glass tube. When the tube 

 was set vertically in the heart of the magnetising coil, the changes of 

 volume were measured by the motions of the liquid meniscus in the 

 capillary tube." " A few experiments were made on the external 

 change of volume of a few of the tubes, which were enclosed in a 

 thin-walled brass tube. The brass tube yielded because of its thin- 

 ness, so that the results were not certain. But there was no doubt 

 that with the specimens of iron tried there were large changes of 

 volume." 



The changes observed by Knott in the interior volume appear in 

 the case of a tube of large bore to have been of the same nature as 

 those in my unannealed ring ; while with a tube of smaller bore they 

 rather resembled the changes exhibited by the annealed ring. His 

 published investigations are, however, only of a preliminary character, 

 and it is not at present possible to make a satisfactory comparison 

 between his results and my own. But he was undoubtedly the first 

 to show that magnetisation is generally attended by considerable 

 changes of volume. 



III. "Note on the Possibility of obtaining a Unidirectional 

 Current to Earth, from the Mains of an Alternating Current 

 System." By Major P. CARDEW. Communicated by LORD 

 KELVIN, P.R.S. Received May 10, 1894. 



In carrying out some tests on the high-pressure alternating current 

 system of the Metropolitan Electric Supply Company Ltd., of a com- 

 bination intended to act as an indicator of leakage to earth, the 

 existence under certain conditions of an excess of current in one 

 direction to earth by leakage through the dielectric of the cables, or 

 through small faults therein, has been demonstrat !. The com- 

 binations and connexions used are shown in fig. 1, where A is the 

 alternating current generator, M! and M 2 the distributing mains, 

 TT the transformers, B a battery of a few Leclanehe cells, G a 



* 'Edin. Roy. Soc. Proc.,' 1891, p. 315; 1892, pp. 85, 249; 'Brit. Assoc. Rep.,' 

 1892, p. 659. The quotations are from the latter. 



H 2 



