TRANSACTIONS OF THE SECTIONS. 249 



was obtained from observations on short lengths of cable immersed in water at vari- 

 ous temperatures. 



The gutta percha covering the long cables was used in alternate layers, with a 

 varnish known as Chatterton's compound. 



The short lengths, tested in water of various temperatures, were three in number, 

 and will be called numbers 1, 2, and 3. The length of each was one knot. The 

 diameter of the gutta percha, in numbers 1 and 2, was 0'3 in., covering a single 

 copper wire of - 06 in. diameter. 



No. 3 was a knot of the Red Sea core : the diameter of the gutta percha was0"34 in., 

 covering a copper strand formed of seven wires, each O - 038 in. diameter. 



The coils were placed in a felted tub, and were covered with water of the desired 

 temperature for several hours before the experiments were made. 



The loss or escape of electricity was measured on a delicate zinc galvanometer. 



Separate tests were made with the positive and negative poles, and at each test 

 five readings were taken ; the first one minute after the application of the current, 

 the others at successive intervals of one minute. 



On coil No. 1, covered with Chatterton's compound and gutta percha, there was 

 a marked difference between the tests made with positive and negative currents ; 

 whereas in coil No. 2, covered with pure gutta percha, there was no difference be- 

 tween those tests from 50° to 75° Fahrenheit. 



In both coils and at all temperatures the loss decreased rapidly during the first 

 two minutes after application of the battery; this decrease continued till the fifth 

 minute, when a minimum was nearly attained. This effect in coil No. 2 was regular, 

 and between 50° and 80° Fahrenheit was not affected by a change in the size of the 

 current. 



In coil No. 1 the decrease of loss was regular and nearly constant when the zinc 

 pole was connected with the coil ; whereas irregular results were obtained when the 

 copper pole of the battery was so connected. 



The extra resistance or decrease of loss was still more marked in coil No. 3, where 

 the gutta percha is of larger diameter. In this coil the loss decreased 30 per cent, 

 in the interval separating the first and fifth minute. 



The same phenomenon (decrease of loss) was observed on the long cables. 



The insulation of a sound gutta percha covered wire is therefore improved by 

 the application of either a positive or a negative current. It also appears that it is 

 most necessary, in testing the insulation of a cable, to record the time separating the 

 observation and the first application of the battery. 



The phenomenon of decreased loss or extra resistance is observed, whether the 

 cable is dry or immersed in water. 



The annexed Tables I. and II., showing the relative loss from the two coils at differ- 

 ent temperatures, give the result of the experiments as deduced from curves which 

 result from the observations after all due corrections have been made for loss on 

 connexions and varying electromotive force. 



The numbers in the Tables give no absolute measurement, but only the relative 

 loss at the various temperatures. 



At 65° the two coils 1 and 2 test much alike. At high temperatures pure gutta 

 percha rapidly deteriorates ; at low temperatures it has the advantage. 



The irregularity of the copper tests of No. 1 lead to some suspicion of a chemical 

 action between the various substances in contact. 



Table No. III. contains the results of similar experiments on coil 3. 



The absolute resistance of the insulating cover was obtained by comparison of the 

 current flowing through the gutta percha, and that flowing through a coil of known 

 resistance, or through the copper core itself. The actual resistance of the insulating 

 cover being known, this formula, due to Professor Thomson, was used to determine the 



specific resistance, or the resistance of a cubic foot : x = R-?ZL_, where R = resist- 



log— 

 a b 



ance of cylindrical coating, 1= length of wire tested, y = ratio of the diameter of the 



copper core to that of the gutta percha covering, x = specific resistance of the 

 material. 



