30 
PEOFESSOE E. EUTHEEFOED AND ME. E. K. McCLUNG 
When this is the case, the amount of heat, H, supplied per second by the rays is 
equal to the amount of heat generated by the current: 
H = ‘24 R gramme calories, 
where i = current through the grid, 
and R = resistance of the grid. 
The heating effect due to the rays was small and consequently care had to be 
taken to avoid disturbances of the balance due to outside causes. The grids were 
enclosed in a lead vessel with an aluminium window in front of the grid A. A thick 
covering of felt completely enclosed the lead vessel. Between the bulb and the 
grid there was one plate of aluminium L millim. thick and two sheets of thin 
aluminium, besides the felt covering. A lead screen in addition could be placed 
over tbe bole H. When the hole was covered thus with the lead plate, there was 
no disturbance of the zero, showing that the rays falling on the grid were responsible 
for the heating effect and the rays alone. 
In practice it was found necessary to remove the sensitive astatic galvanometer 
employed a considerable distance away from the induction coil before the magnetic 
disturbances due to it were negligible. This necessitated additional leads, and in 
consequence more troublesome changes of the balance. For the most part, however, 
the changes of the balance point were gradual and, if necessary, could be accurately 
allowed for during the short time the grid was exposed to the rays. 
Two observers were required, one to start and stop the rays and to note the electro¬ 
meter deflections, and the other to observe tbe galvanometer deflections. With the 
aid of a simple system of signals, the experiments presented no serious difficulty. 
In order that the rays should be, as far as possible, of constant intensity when 
falling on the grid, the hole in the lead plate between the bulb and the grid was 
covered with a lead screen, operated from a distance, during the first 15 seconds 
after the rays were turned on. By means of a cord the screen was suddenly removed 
and the rays stopped after a definite time by breaking the current. 
The following table is an example of a succession of observations extending over 
several hours :— 
Time of exposure to the rays = 30 seconds. 
Testing current through the grids = '04 ampere. 
Deflection of 
galvanometer in millims. 
17'0 
18-6 
18-4 
17'8 
Deflection of 
electrometer in scale divisions 
per second. 
5-17 
5-53 
5-33 
5-27 
Mean value = 17'9. Mean deflection = 5'32 
