ON STANDARDS OF ELECTRICAL RESISTANCE. 153 
between them. This force may be variable and measured by the torsion of a 
wire, as in Thomson’s reflecting electrometer, or it may be constant, and the 
electromotive forces producing it may be compared by measuring the distance 
required in each case between the two electrified bodies to produce that 
constant force. The latter arrangement is adopted in Professor Thomson’s 
portable electrometer, first exhibited at the present meeting of the Association. 
The indications of a gauge or electrometer not in itself absolute may be re- 
duced to absolute measurement by multiplication into a constant coefficient. 
45. Practical Measurement of Electric Resistance—The electrostatic resist- 
ance of a conductor of great resistance (such as gutta percha or india rubber) 
might be directly obtained in the following manner :—Let a body of known 
capacity, s (40), be charged to a given potential, P (47), and let it be gradually 
discharged through the conductor of great resistance, r. Let the time, ¢, be 
noted at the end of which the potential of the body has fallen to p. The rate 
in SS t P 
of loss of Bcobricrly will then be 2 - Hencep=P, rand = log, e Hence 
t 
ERT EI SS RGR EC 
P 
from which equation x can be deduced, if s, ¢, and the ratio = be known, ¢ can 
be directly observed, s can be measured (40), and the ratio sa can be measured 
by an electrometer (44) in constant connexion with the charged body. This 
ratio can also be measured by the relative discharges through a galvanometer, 
first, immediately after the body has been charged to the potential P, and 
again when, after having been recharged to the potential P, it has, after a time 
t, fallen to potential p. (This latter plan has long been practically used by 
Messrs. Siemens, although the results have not been expressed in absolute 
measure. 
Unfortunately, in those bodies, such as gutta percha and india rubber, the 
resistance of which is sufficiently great to make ¢ a mensurable number, the 
phenomenon of absorption due to continued electrification* so complicates the 
experiment as to render it practically unavailable for any exact determination. 
The apparent effect of absorption is to cause 7, the resistance of the material, 
to be a quantity variable with the time ¢, and the laws of the variation are 
very imperfectly known. 
46. Experimental Determination of the Ratio, v, between Electromagnetic and 
Electrostatic Measures of Quantity—In order to obtain the value of v, it is 
necessary and sufficient that we should obtain a common electrostatic and 
electromagnetic measure of some one quantity, current, resistance, electro- 
motive force, or capacity. There are thus five known methods by which the 
value can be obtained. 
1°. By a common measure of quantity. Let a condenser of known capacity, 
s, be prepared (40). Let it be charged to a given potential P (47). Then 
the quantity in the condenser willbe sP in electrostatic measure. The 
charge can next be measured by discharge through a galvanometer (25) in 
electromagnetic measure. The ratio between the two numbers will give the 
value of v. The only difficulty in this method consists in the measurement 
* Vide Transactions of British Association, 1859, p. 248, and Report of the Committee of 
Board of Trade on Submarine Cables, pp. 136 & 464. 
