ON ENERGY OF RONTGEN AND BECQUEREL RAYS, ETC. 
41 
The tubes were first adjusted so that the rays caused no movement of tlie electro¬ 
meter needle. The tube, A, was then rapidly exhausted by means of a Fleuss pump. 
The intensity of the rays after emerging from tlie tube was thus greater than for 
the tube A' on account of the less absorption, and the electrometer therefore showed 
a deflection. If the tube A' were exhausted and the tube A filled with air, the 
electrometer gave a deflection in the opposite direction. If the end of one of the 
tubes was closed with a thick lead plate so that no rays could get through, then the 
rate of movement of the electrometer needle corresponded to the intensitv, I, of the 
rays after emergence from the other tube. 
If \ is the coefficient of absorption of the rays in the gas, then the intensity of the 
rays after passing through a distance d of the tube is of its value if there had 
been no absorption. 
Since the currents between C and D and l^etween C' and D' are proportional to tlie 
intensities of the radiations between the jilates, then 
Difference between currents I — 
Total current I 
— I _ 
= \d, if hJ is small. 
In order to determine Xd, we thus reipiire the ratio of the number of division,^ jier 
second, given Iiy tlie electrometer needle from the balance when one of them is 
exhausted, to the number per second when the end of tlie tube containing the gas is 
covered with a thick lead plate. 
The following table gives the results for air at pressures in one tube ranging 
from '5 of an atmosphere to 3 atmospheres, the other tube being exhausted ;—■ 
Difference of pressures 
in atmospheres. 
Number of divisions in 
5 secs, with one tulje 
screened. 
Deffection from balance 
in 20 secs. 
XJ. 
o 
•0187 
1 
160 
21-8 
•0.34 
•) 
169 
48-0 
•071 
•> 
o 
172 
70-0 
•102 
The above resiflts are the mean values of a series of measurements. The results 
for '5 of an atmosphere were obtained at a different time from the others and with a 
different sensitiveness of the electrometer. The table shows that the absorption of 
the rays in the gas is approximately proportional to the pressure. 
The value of d was 118 centims. 
The value of X for different pressures is thus given by the following table :— 
VOL. CXCVI.—A. 
o 
