8 Cady — On the Energy of the Cathode Rays. 



probable that eq. (4«) is approximately correct, although the 

 limiting value of the ratio is considerably smaller than unity. 

 Still owing to the sources of error in the use of the thermo- 

 element, no great dependence can be placed upon absolute 

 values. It was therefore determined to measure the energy Q 

 in a surer manner, by Kurlbaum's bolometric method.* 



Observations with Bolometer. 



A surface-bolometer forms one arm of a Wheatstone bridge; 

 suppose the bridge balanced, and a current of intensity I x 

 flowing through the bolometer. Let the resistance of the bolo- 

 meter under these conditions be w. If now an amount of heat 

 Q per second be supplied by the cathode rays to the bolometer, 

 the galvanometer in the bridge will suffer a certain deflection. 

 Suppose the same deflection caused by increasing I x to 1^ 

 the cathode rays no longer acting. Then from Kurlbaum's 

 formula, 



Q = w 1 ^(I*-I*) watts, 



where 1 and w are expressed in amperes and ohms respectively. 

 If the four bridge resistances are equal, then I=%c, where 

 c is the current in the battery branch, and 



c 

 Q — \w—-{c^ — c x ) watts. 



As it was not practicable to make both deflections exactly 

 equal, the following method was adopted : 



1. galv. defl. a 1 with current c l5 rays falling. 



2. " " a 2 " " c 2 , " deflected. 



We thus get the following equation : 



a 2 c x v 2 



The thermoelement formerly used (S, fig. 1) was replaced 

 by a bolometer, whose terminals were led out through the 

 plate P. The bolometer was made after Lummer and Kurl- 

 baum's process, f with a few modifications. In the first obser- 

 vations it consisted of a platinum foil about O001 mm thick, 

 mounted with mica insulation upon a brass frame. The form 

 is shown in B (fig. 2). Care was taken that the entire bolo- 

 metric resistance should be reached by the rays ; also that all 

 cathode rays entering the Faraday cylinder that did not strike 



*F. Kurlbaum, Wied. Ann., lxv, p. 746, 1898. 



f 0. Lummer and F. Kurlbaum, Wied. Aun., slvi, p. 204, 1892. 



