32 
PROFESSOR E. RUTHERFORD AND j\JR. R. K. McCLUNG 
Tims the amount of heat supplied to the grid per second by the rays 
17‘9 
= —^ X (•02)® X '734 —- •000141 gramme calorie. 
In the first stage of the investigation a null method was employed to measure the 
heat of the rays, but unexpected difficulties arose, and the method was abandoned. 
The battery current was kept steadily flowing through the grids, and the balance 
obtained. During the time the rays were on, a portion of the current through the 
grid was shunted through a resistance of known value. The value of the shunt 
resistance was adjusted until there was no change ol the balance immediately after 
tbe rays were sto^Dped and tbe shunt circuit broken. 
It was difficult, however, to obtain satisfactory results, partly on account of the 
inconstancy of the rays, but cbiefly on account of the slight difference of heating 
effect of the two grids for equal currents. The strength of current through the grid 
was generally •QI of an ampere, and with this current the inequality of the grids was 
immediately seen by a change of balance, when the current was apjdied for some 
time. The addition of a shunt to one grid caused a variation of current through both 
grids, and the change of temperature, due to inequality of the grids, introduced an 
error which was not negligible compared with the small heating effect of the rays. 
The method was not so luj^id or certain a the one finally employed. 
Measurement of Heating Effect by Thermojnle. 
Some experiments were made to see it a thermopile was suitable for a measure of 
the heating effect of X rays. The only thermopiles in the laboratory were of the 
ordinary solid type of 65 bismuth antimony couples. The thermopile was placed 
inside a metal tube covered with aluminium at one end and a rock-salt plate at the 
other. With a sensitive low-resistance galvanometer a deflection of 15 millims. could 
be obtained in 30 seconds. The rate of siq^ply of heat was standardised by using a 
standard Hefner amyl-acetate lamp, the total radiation from which has been deter¬ 
mined in absolute measure by Tumliez."^^ It was observed that the thermopile, 
when exposed to the X rays, took up its final temperature very much more slowly 
than when exjDosed to the radiation from the lamp, and tlrat the results obtained 
differed considerably from the bolometer method. The cause of the discrepancy lies 
in the unsuitability of a solid thermopile for measurements on X rays. The radiation 
from the lamp falling on the lampblack coating of the thermopile is absorbed at the 
surface of the metal, while the X rays penetrate a distance of the order of 1 millim. 
before much of the energy is absorbed. On account of this, the maximum rise of 
temperature near the surface of the junction on which the E.M.F. depends is less 
* ‘ Wied. Annal.,’ vol. 38, p. 6t0. 
