Maximum Frequency of X Rays from a Coolidge Tube. 341 



of wave-lengths employed in the experiment, viz. from 

 X=14xl0- 9 cm. to \=lxl0" 9 cm. 



Jt was hoped at the same time to make a systematic 

 examination of the X-ray spectrum of the radiations, and to 

 determine if possible the voltage at which the spectrum-lines 

 appeared. Barnes in an accompanying paper * has given 

 the wave-lengths of the spectrum-lines observed with the 

 Coolidge tube. On account of the thickness of the glass 

 of the vacuum-tube, the intensity of the issuing " L ' 

 characteristic radiation was weak when examined electrically 

 or photographically, but permitted of the determination of 

 the wave-lengths of the stronger lines. No evidence of 

 other well marked lines was noted in the region of higher 

 frequencies, but the main part of the spectrum appeared to 

 be continuous with the crystals employed. As will be seen 

 later, the experiments on absorption of the radiation showed 

 that the a end " frequency increased regularly with increase 

 of voltage. No evidence was obtained that the maximum 

 frequency for different voltages varied by jumps, such as 

 might be expected if the issuing radiations were mainly 

 confined to a few waves of definite frequency. 



Arrangement of the Experiment. 



The energy of the electrons striking the anticathode in the 

 high vacuum of the Coolidge tube depends only on the 

 voltage applied. In order to investigate the effect of elec- 

 trons at definite speed, it was necessary to employ a constant 

 voltage delivered by an influence-machine in place of the 

 variable voltage due to an induction-coil or transformer. 

 For this purpose, the only machine available was a large 

 AVimshurst of 12 plates of diameter 71 cm. This had been 

 presented to the Department about 15 years previously, and 

 was in some respects not nearly so well suited for the 

 experiment as one of the more modern types of high-speed 

 machines. The Wimshurst, of which 10 plates had survived, 

 was run by a motor, and after a month's fairly continuous 

 running, four more of the plates cracked and were removed. 

 This proved fortunate, for it was found that the machine 

 with six plates gave nearly the same maximum voltage and 

 delivered nearly the same current as in the beginning, and 

 ID addition ran much more steadily. 



In order to reduce the losses to a minimum, all the con- 

 ductors consisted of light metal tubing 27 cm. in diameter 

 with rounded ends. These passed through large paraffin 

 insulators to the Coolidge tube T, which was placed in a box 



* Infra, p. 368. 



