SCIENCE AND PRACTICE. 365 



ance between K and a, which is removed when the steady 

 deflection due to leakage, or conduction of the insulator, is to 

 be observed. The details of these arrangements were given 

 in the description of Messrs. Siemens' testing-board. 



80. Resistance of Insulating Materials under Pressure. 

 The arrangements made by Mr. Reid at the gutta-percha 

 works enabled Mr. C. W. Siemens to have the core of his 

 cable tested under a hydraulic pressure of 280 atmospheres. 

 He took advantage of this to make some interesting experi- 

 ments on the electrical behaviour of gutta-percha, india- 

 rubber, and a combination of both, under high pressure. The 

 results of these experiments, which the writer carried out 

 under Mr. Siemens' direction, were read before the British 

 Association at their recent meeting at Newcastle. The core 

 of the Malta- Alexandria cable was tested under a pressure of 

 600 Ibs. per square inch, and from these tests it was observed 

 that the resistance of insulation increased, under this pressure, 

 to the amount of 14 per cent. ; or, more generally, that the 

 resistance, R^, of a coil of this cable, under the pressure of 

 p Ibs. per square inch, whose resistance under atmospheric 

 pressure at the same temperature, was E, could be very nearly 

 calculated by the formula. 



R p = R (1 -f- 0,00023^) 



The stronger tank since erected by Mr. Reid enabled the 

 tests of part of the Carthagena-Oran core to be carried to 

 300 atmospheres. The resistance of the insulation was 

 observed at different stages of the pressure, between vacuum 

 and 300 atmospheres, advancing each time 75 atmospheres. 

 The results of the tests with gutta-percha covered wire showed 

 that the resistance increases with the pressure, and that the 

 curve is not approximately a straight line, as the above 

 formula expresses it, but that it is slightly convex to the 

 axis of x, when the ordinates of a graphic system (Fig. 168) 

 represent the resistances, and the abscissae the pressures, in 

 atmospheres. Wires insulated with india-rubber gave quite 

 opposite results, the resistance decreasing as the pressure was 

 increased, and the curve being somewhat concave to the base 

 line. A wire insulated first with india-rubber and then, 



