On the Use of Price's Guard-Wire in Insulation Tests. 343 



intensity of magnetization was supposed to be simply propor- 

 tional to the magnetizing force. In fact, the theory is still in 

 its infancy, so that there are ample grounds for further 

 development and research. 



Physical Laboratory, University of Tokyo, 

 December 20th, 1899. 



XXXI. Some Developments intlie Use of Price* s Guard-Wire 

 in Insulation Tests. By Prof. W. E. Ayrton, F.R.S., and 

 T. Mather *. 



EVERYONE who has to test very high resistances, such as 

 those of short lengths of cable and of good joints in 

 insulated wires, cannot fail to appreciate the immense advantage 

 of using Price's " Guard- Wire ; ' f in eliminating errors due 

 to surface leakages. Continued experience confirms the 

 opinion expressed by one of us in the discussion on 

 Mr. Appleyard's paper in 189(5 ; and in our third paper on 

 Galvanometers, read before the Physical Society in May 1898 J, 

 we showed how the principle could be applied to galvano- 

 meters, shunt-boxes, &c. 



For tests made by the " direct deflexion " method the 

 " guard-wire '' properly applied affords complete protection 

 against surface leakage where the ends of the cable tested 

 are near the galvanometer, so that it is possible to have the 

 wire connecting the conductor of the cable with the galvano- 

 meter terminal ft air insulated/' A difficulty, however, 

 arises when the ends of the cable are at a considerable 

 distance from the testing instrument ; this may render " air 

 insulation "of the lead impossible, and so the leakage from 

 the lead must be measured and allowed for. As this " lead 

 leakage " may be as large as or larger than that through the 

 mass of the insulation under test, the convenience and accuracy 

 of the measurement are somewhat impaired. It therefore 

 occurred to us that a guard-wire applied to the lead along its 

 entire length would prove of great value in cases such as 

 the one under consideration. The most effective as well as 

 the simplest way of doing this is to use a concentric wire 

 to connect the cable and galvanometer, the inner of the 



* Communicated by the Physical Society: read January 26, 1900. 



f Electrical Review, vol. xxxvii. p. 702 (1895). See also Appleyard 

 on "Dielectrics," Phil. Mag. xlii. p. 148 (1896) ; Proc. Phys. Soc. vol. xiv. 

 pp. 257, 264. 



X See Phil. Mag. Oct. 1898, p. 368, and Proc. Phys. Soc. vol. xvi. 

 p. 191. 



