SCIENCE AND PRACTICE. 357 



idea of the magnitude of the fault through which the current 

 found a complete circuit. 



The instruments used for this purpose were of a rough 

 nature, badly insulated, and insensible to currents of small 

 intensity. But notwithstanding the obvious insufficiency of 

 this method of testing, it did not entirely give place to a 

 more scientific way until the date when the Malta- Alexandria 

 cable was begun. The Government tests applied to this core 

 began, however, the work of civilisation; the insulation 

 resistances and the copper resistances were expressed in one 

 and the same unit, and were thus made directly comparable 

 with each other ; the dependence of the resistance of the con- 

 ductor upon the conducting power of the metal used and upon 

 its dimensions, and the dependence of the resistance of the in- 

 sulating covering upon the specific resistance of the material 

 and its dimensions being known, these resistances were calcu- 

 lated and the results compared with those found by actual 

 experiment; and thus the electrical conditions of the cable 

 were judged by the agreement of the results found with those 

 expected. 



Dr. Werner Siemens was the pioneer who began this very 

 serviceable work, and carried it through ; and, after the first 

 prejudice at innovation had been got over, electricians, one after 

 the other, fell into the same way of thinking and of measuring. 



In manufacturing the core of a submarine cable, as much 

 care is devoted to the selection of gutta-percha of high specific 

 insulation as of copper of high conducting power. Both are 

 only to be attained by freeing the commercial materials from 

 impurities. 



The Gutta-percha Company, of Wharf Road, have suc- 

 ceeded signally in the production of first-rate insulation. 

 The way they secure this is by selecting the best gum, and, 

 after the process of cutting the imported blocks into small 

 shavings and masticating it at the temperature of boiling 

 water, of straining the plastic material through sieves of fine 

 wire gauze. By this operation, almost all the natural im- 

 purities of the gum are removed, and the substance rendered 

 homogeneous and of low conducting power. 



