Mr. Jenkyn on the Insulating Properties of Gutta Percha. 77 
The cables were not wet; but direct experiment proved that cover- 
ing a sound iron-covered cable with water has no effect on the loss. 
The details of this experiment are given in the paper. 
The résistance G was obtained in the following manner. The 
copper conductor of the cable to be tested was arranged so as to form 
a complete metallic are with a battery of 72 cells and a tangent gal- 
yanometer : the deflection on this galvanometer was read and entered 
as the continuity test. Deflections were then read on the same gal- 
vanometer with the battery and several known resistances in circuit, 
for the purpose of measuring the resistance and electromotive force 
of the battery, in the manner described by Pouillet. The deflection 
caused by the loss was next read on a second tangent galvanometer : 
the same battery was used. This deflection was entered as the insu- 
lation test. The temperature of the tank containing the cable was 
observed by means of a thermometer inserted in a metal tube, ex- 
tending from the circumference into the mass of the coil. 
The relative delicacy of the galvanometers was ascertained by 
experiment, or, in other words, the coefficient was found by which 
the tangents of the deflections of the first were multiplied to render 
them directly comparable with the tangents of the deflections of the 
second galvanometer. 
The resistances of the galvanometer coils, of the artificial resistance 
coils, and of the copper conductor of the cable were measured by 
Wheatstone’s differential arrangement. Special experiments were 
made by means of this differential arrangement to determine the 
change of resistance of the copper conductor in the cable, produced 
by a change of temperature. 
The equation (No. 2) R=7r(1+0-00192¢) gives the value of the 
resistance R of the copper wire at any temperature ¢+ a in function 
of the resistance 7 at any temperature a (Fahrenheit). The length 
and temperature of any coil being known, the resistance of the 
copper wire was thus at once obtained from the resistance of one 
knot at 60°, which was very carefully determined. 
Now let G=resistance of cylindrical coating. 
D=deflection called the continuity test. 
d=deilection called the insulaticn test. 
C=coefiicient expressing the relative delicacy of the two 
galvanometers. 
BR=resistance of the battery. 
'T,=resistance of the coil of first galvanometer. 
T,=resistance of the coil of second galvanometer. 
C tan DX (BR+T,4+) WV 
Then (aise alla alta a at (3) 
tan d 
G having been thus obtained in any desired units, S, the specific 
resistance of the material, can be at once obtained by equation No. 1, 
which appears from several experiments to give constant values for 
S when calculated from cables of different dimensions. In extreme 
eases, however, the influence of extra resistance would render the 
formula defective, especially after continued application of the cur- 
