4G 
PROFESSOR E. RUTHERFORD AND MR. R. K. McCLUNG 
deflection of the electrometer was taken 1)}^ another. In the later experiments the 
capacity of the electrometer and connections was oV of the capacity added, and 
the total capacity of the circuit was '00257 of a microfarad. 
The following is an example of the determination of i in electrostatic units :— 
1 Clark cell of 1'434 volts E.M.F. gave 57'0 divisions on the electrometer scale. 
O 
The deflection of the electrometer due to the passage of the rays for 30 seconds Avas 
160 divisions. 
i = quantity of electricity ^^er second 
00257 
10 « 
160 
1-434 
57 
X 3 X 109 
= 1'03 electrostatic units. 
Determination of the Absoi'ption of the Rays in the Bolometer, the Aluminium 
Window, &c. 
The values ot and p.i were determined electrically by utilising the discharge 
cylinder of fig. 6. The rate of discharge in the cylinder was observed with the grid 
l)efore the hole in the lead plate and then without the grid. The ratio of the 
currents in tlie two cases is proportional to the ratio of the intensities, since the 
ionization is proportional to the intensity of radiation. A inean value of tlie ratio for 
different portions of the grid was taken, and it was found that p^ = '453. It is thus 
seen that the intensity of the radiation was cut down to a little more than lialf in passing 
through the platinum grid. Tlie value of p.i was determined in a similar manner. The 
intensity of the rays in this case was cut down more in consecjuence of passing through 
a thickness of felt, an aluminium window of '1 centim. thickness, and a thin layer of 
aluminium, as well as the platinum grid. The value found for this ratio was pr^^ "31. 
The absorption of the rays in the mica frame appears in the values of p^ and p.y 
Tlie absorption, however, was small and practically negligible in any case. 
Dimensions of the Apparatus and Yahies of Constants. 
The area of the rectangular hole through winch the rays passed into the discharge 
cylinder, and from which S was calculated, was 7'1 sq. centims. The length of 
the centre plate in the discharge cylinder was 12'06 centims. For most of the 
experiments, the distance d of the grid from the source of rays was 26'0 centims., 
and the distance of the hole in the lead plate from the source was 45'2 centims. 
Area of platinum grid was 92'6 sq. centims. 
Mean value of X — '000279. 
Value of e = 6'5 X lO"^^ electrostatic unit. 
The correction for the absorption in air between the grid and the discharge 
cylinder was negligible. 
