76 METHODS OF MEASUREMENT [CH. 
or by the movement of the spot of light on a scale in the usual 
way. If the needle is damped so as to give a uniform motion 
over the scale, the rate of movement of the needle, 7.e. the number 
of divisions of the scale passed over per second, may be taken as 
a measure of the current through the gas. The rate of movement 
is most simply obtained by observing with a stop-watch the time 
taken for the spot of light, after the motion has become steady, to 
pass over 100 divisions of the scale. As soon as the observation is 
made, the plate B is again connected with earth, and the electro- 
meter needle returns to its original position. 
In most experiments on radio-activity only comparative measures 
of saturation currents are required. If these comparative measures 
are to extend over weeks or months, as 1s sometimes the case, it 1s 
necessary to adopt some method of standardizing the electrometer 
from day to day, so as to correct for variation in its sensibility. 
This is most simply done by comparing the current to be measured 
with that due to a standard sample of uranium oxide, which is 
placed in a definite position in a small testing vessel, always kept 
in connection with the electrometer. Uranium oxide is a very 
constant source of radiation, and the saturation current due to it 
is the same from day to day. By this method of comparison 
accurate observations may be made on the variation of activity of 
a substance over long intervals of time, although the sensibility 
of the electrometer may vary widely between successive measure- 
ments. 
55. Construction of electrometers. As the quadrant 
electrometer has gained the reputation of bemg a difficult and 
uncertain instrument for accurate measurements of current, 1t may 
be of value to give some particular details in regard to the best 
method of construction and insulation. In most of the older types 
of quadrant electrometers the needle system was made unneces- 
sarily heavy. In consequence of this, if a sensibility of the order 
of 100 mms. deflection for 1 volt was required, 1t was necessary to 
charge the Leyden jar connected to the needle to a fairly high 
potential. This at once imtroduced difficulties, for at a high 
potential it is not easy to insulate the Leyden jar satisfactorily, or 
to charge it to the same potential from day to day. This drawback 
