411 



appear between the temperatures of 50 and 60 ; but the resistance 

 is somewhat greater than with the negative current. The extra 

 resistance due to continued electrification is unchanged by a change 

 in the sign of the current. Above the temperature of 63 great 

 irregularities occur in the observations, which could not even be 

 included in regular curves. The difference in the resistance of the 

 gutta-percha coating when the copper is positively and negatively 

 electrified, may be caused by the .contact between the resinous com- 

 pound and the copper : no such difference was observed when pure 

 gutta percha was in contact with the copper. 



The curves resulting from the experiments on the second coil, 

 which was covered with pure gutta percha, present an entirely dif- 

 ferent character from those resulting from the first coil. The copper 

 and gutta percha were of the same size in these two coils. The re- 

 sistance of pure gutta percha at low temperatures is greater than 

 that of the compound covering. At 65 the resistance of the two 

 coverings is equal ; at higher temperatures the resistance of pure 

 gutta percha diminishes extremely rapidly. The curves obtained 

 with positive and negative currents are identical up to about 75 ; a 

 slight difference occurs above this temperature, which may have been 

 accidental. The extra resistance is less with pure gutta percha than 

 with the compound ; it increases slightly at high temperatures, and 

 is not affected by a change in the sign of the current. 



The curves derived from the experiments on the third coil, which 

 contained a smaller proportion of Chatterton's compound than the 

 first coil, appear in some respects intermediate between those derived 

 from the first and second coils. The extra resistance due to con- 

 tinued electrification was still greater in this coil than in the others. 

 40 per cent, of the entire resistance is at 70 due to this cause. 

 This increase is believed to be due to the greater mass of gutta 

 percha used in covering this coil, which was of larger dimensions 

 than the two others. 



PART II. 



Professor Thomson has supplied an equation expressing the law 

 which connects the resistance of a cylindrical covering, such as that 

 of a cable, with the resistance of the unit of the material forming the 

 covering. 



