lt] METHODS OF MEASUREMENT 87 
rods at each end. The capacity is given approximately by the 
formula 
I 
C=——, 
2, log. a 
where 6 is the internal diameter of D,a the external diameter of C, 
and J the length of the tubes. 
The following method can in some cases be used with advantage. 
While a testing vessel is in connection with the electrometer, a 
sample of uranium is placed on the lower plate A. Let d, and 
d, be the number of divisions passed over per second by the needle 
with and without the standard capacity im connection. 

GANG Gh 
Then LOA a. 
ds 
and C= Ci Gh =O 6 
This method has the advantage that the relative capacities are 
expressed in terms of the motion of the needle under the actual 
conditions of measurement. 
64. Quartz piezo-electrique. In measurements of the 
strength of currents by electrometers, it 1s always necessary to 
determine the sensibility of the instrument and the capacity of the 
electrometer and the apparatus attached thereto. By means of the 
quartz piezo-electrique devised by the brothers MM. J. and P. Curie’, 
measurements of the current can be made with rapidity and 
accuracy over a wide range. These measurements are quite inde- 
pendent of the capacity of the electrometer and external circuit. 
The essential part of this instrument consists of a plate of 
quartz which is cut in a special manner. When this plate is 
placed under tension, there is a liberation of electricity equal in 
amount but opposite m sign on the two sides of the plate. The 
plate of quartz AB (Fig. 19) is hung vertically and weights are 
added to the lower end. The plate is cut so that the optic axis of 
1C. R. 91, pp. 38 and 294, 1880. See also Friedel and J. Curie, C. R. 96, 
pp. 1262 and 1389, 1883, and Lord Kelvin, Phil. Mag. 36, pp. 331, 342, 384, 414, 
453, 1893. 
