76 Royal Society :— 
constant. At 60° the resistance increases about 20 per cent. im five 
minutes from this cause. With a positive current, similar results 
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. ‘I'he extra resistance 1s 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. 
Parr 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. 
Let S be the specific resistance of the material, or the resistance 
of a bar one foot long, and one square foot in section; let G be the 
resistance of the cylindrical cover of a length of cable L; let 5 be 
the ratio of the external to the internal diameter of the covering ; 
then 
27LG 
= Bo ee 
log Z 
The resistance G was calculated from cables of various lengths, 
lying in iron wells at the works of R. S. Newall and Co., Birkenhead. 
