26 
PROFP]SSOR E. RUTHERFORD AND MR. R. K. McCLUNG 
Heating Effect of X Rays. 
Experiments on the heating eflPect of Rontgen radiation have been made by Dorn.* 
The rays were partly absorl)ed in metal foil placed in one bulb of a difterential air- 
thermometer. The heat absoiTed by the metal was communicated to the gas and the 
resulting change of volume observed. In order to obtain a measure of the heat 
supplied, the heating effect due to a current in a wire placed inside the bulb was 
observed. 
MoffatI has deduced the energy of X rays from photometric comparisons of a 
fluorescent screen with the Hefner amyl lamp, assuming the efficiency of a fluorescent 
screen excited by X rays as a source of light. Knowing the value of the energy of 
the visible light of the Hefnei' standard, the heating effect of the rays can be 
deduced. 
In determining the heating effect of the rays, difficulties arise from which measure¬ 
ments of the iieating effect of weak sources of visil)le light are free. In the first case, 
the inconstancy of an X-ray bulb as a source of radiation for measurements extending 
over long Intervals is always a cause of trouble. In the second place, the X rays 
are only slightly absorl)ed in thin metal foil, while light rays are completely absorbed 
at the surface'of thin metal coated with lampblack. Only a small portion of the 
energy of the rays is absorbed in passing through thin metal foil, and in consequence 
a bolometer like Langley’s, where the change of resistance of a very thin metal 
sheet, due to heat sup})lied by the rays, is observed, is not very suitable for measure¬ 
ments on the energy of X rays. Ordinary thermopiles are o 2 )en to grave objections, as 
will be explained later in this paper. 
In order to measure the heating effect of the rays, a specially designed platinum 
Ijolometer was employed, and the heating effect was determined from the change of 
resistance of the platinum. 
Description of Bolometer. (Fig. 1.) 
A platinum strip, about 3 metres long, A centim. wide, and '003 centim. thick, 
was wound on an open mica frame made as light as was compatible with rigidity. 
The frame was 10 centims. square, and of a shape shown in fig. I («). The platinum 
strip was wound round and round the frame, the strips on the front of the frame 
partly overlapping the corresponding ones at the back, but not touching tliem. The 
platinum strip (fifteen complete turns in all) was held in position l)y notches in the 
side of the mica frame, and the distance between each turn of the strip was 1 millim. 
* ' tVied. Aniial.,’ vol. 63, p. 150. 
t ‘ Roy. Soc. Edin. Proc.,’ 1898. 
