414 



of pure gutta percha, gave very regular and complete results. An 

 abbreviation of the Table is annexed. 



It was remarked that in the tests of the cable in the iron tanks, 

 the resistance after five minutes' electrification was invariably 

 greater with zinc than with copper to cable, whilst the reverse was 

 the case with the single knot covered by water. The length of the 

 cable, and the condition of immersion or non-immersion, have pro- 

 bably some influence on the phenomenon of extra-resistance. This 

 phenomenon appears to the author to be of much importance, and 

 to demand further investigation. 



The values of G were also calculated from the daily tests of the 

 cables during manufacture at many temperatures. These values 

 agreed with those given in the Tables above described. The general 

 results of the experiments may be summed up as follows. 



The relative loss at various temperatures through pure gutta 

 percha has been pretty accurately determined for all ordinary tem- 

 peratures. To a less extent the same knowledge has been gained 

 concerning two other coatings containing Chatterton's compound. 

 The latter appears superior at high, and inferior at low temperatures. 



Attention has been drawn to the considerably increased resistance 

 which follows the continued electrification of gutta percha and its 

 compounds. Some of the laws of this extra resistance have been 

 determined, and some suggestions made as to the cause of the 

 phenomenon. 



The bounds have been pointed out within which formulae may 

 be used, which consider gutta percha as a conductor of the same 

 nature as metals. 



The resistance of gutta percha has been obtained in units, such as 

 are employed to measure the resistance of metals ; and by the use of 

 Professor Thomson's formula, the specific resistance of a unit of the 

 material has been fixed with some accuracy. 



The resistance of other non-conductors, such as glass and the 

 resins, may probably, by comparison with gutta percha, be obtained 

 in the same units. 



Incidentally, the increase of resistance in copper with increased 

 temperature has been given from new experiments ; arid it has been 

 shown that the insulation of a sound wire-covered cable is little, if at 

 all, affected by submersion. 



