692 REPORT—18638. 
earth for the same length of time; also two such series without pressure, the 
first immediately after taking off the pressure, the other after the cable had 
been three hours to earth. 
, ‘ i eer 
It appears from the coefficients in the columns headed eS AP 2 this Table, 
that pressure has no marked influence on the electrification of gutta percha ; 
the relative amount of electrification being, in all cases, only a function of the 
time during which the current is kept on. The same constancy of electrical 
resistance or electrification was observed in regard to differences of tempera- 
ture by Mr. Fleeming Jenkin as early as 1859 *. 
In the measurements recorded in Table III., the electrification of the cable 
was completed under atmospheric pressure before taking the first reading. 
The pressure was then raised to 75 atmospheres, under which the resistance 
was observed. The pressure was then taken off, and the resistance observed 
again. Readings were then made alternately under atmospheric pressure and 
pressures increased each time by 75 atmospheres. 
It will be observed that the resistance of the insulating covering increased 
on the application of the pressure of 75 atmospheres from 380 to 490 millions, 
and on taking off this pressure did not, as might have been expected, imme- 
diately fall back again to its original value, but only after an interval of 
three hours. 
The coefficients in Tables I., II., and III. are on an average the same as 
those obtained with the core of the Malta-Alexandria cable. 
I may further mention here that the tests were extended to measuring the 
induction of the insulating materials. The results proved that the inductive 
capacity of gutta percha is not affected by pressure. 
Being desirous to ascertain whether the same physical laws indicated by 
these observations were equally applicable to other insulating materials, I 
subjected a length of wire covered with india-rubber to the same test. A 
length of half-a-mile of copper wire, No. 16, B. W. G., which had been covered 
with three coats of masticated india-rubber by the longitudinal process intro- 
duced by me some years ago, was kindly placed at my disposal by the Gutta- 
percha Company, after having been submerged in the canal at Wharf Road 
for a period of about 18 months. At the ends where the core had been exposed 
to the air and light, the india-rubber had turned into a viscid mass, but, on 
raising the coil out of the water, it was found to be perfectly sound, although 
the india-rubber had turned white by absorption of water, and was completely 
covered with vegetable and animal substances. Its electrical resistance, ac- 
cording to the tests of Dr. Esselbach and of Mr. Willoughby Smith, Electrician 
to the Company, was still very high. 
Table IV. contains the resistances of the india-rubber coating; in millions 
of units, after the zinc-current had been kept half-an-hour on, under various 
pressures ; and Table V. the resistances under different pressures read off after 
the current had been on 1, 4, 9, 14, and 19 minutes respectively. 
From the test in the former of these Tables, the surprising fact was demon- 
strated that the electrical resistance of india-rubber decreases as the pressure is 
increased, being the reverse of the behaviour of gutta percha. 
The vertical columns in Table V. corroborate this, whilst the horizontal 
columns (showing the resistance after the current had been active during the 
stated intervals of time under various pressures) indicate this further remark- 
able difference between the two insulating materials, that whereas, in the 
* Report of the Twenty-ninth Meeting of the British Association at Aberdeen in 1859, 
Trans. of Sect, p. 248, : 
