314 Prof. E. Rutherford and Mr. H. Robinson on the 



(3) The agreement between the observed heat emission of 

 the emanation and its products and the value calculated on 

 the assumption that the heat emission is a measure of the 

 absorbed radiations. 



(4) The heating effect due to the /3 and 7 rays. 



It was also of interest to test whether the product 

 radium B, which emits no a, rays but only j3 and 7 rays, 

 contributed a detectable amount to the heat emission of the 

 active deposit. 



Method of Experiment. 



In order to test these points, it was essential to employ a 

 method whereby rapidly changing heating effects could be 

 followed with ease and accuracy. A sufficient quantity of 

 radium emanation was available to produce comparatively 

 large heating effects. It was consequently not necessary to 

 employ one of the more sensitive methods for measuring small 

 heating effects, such as have been devised by Callendar and 

 Duane. 



The general arrangement was similar to that employed in 

 1904 by Rutherford and Barnes for a like purpose. Two 

 equal coils, P, P, fig. 1, about 2*5 cm. long, were made of 

 covered platinum wire of diameter *004 cm. and length 

 100-300 cm., and wound on thin glass tubes of 5*5 mm. 

 diameter. These platinum coils of nearly equal resistance 

 formed two arms of a Wheatstone bridge, while the ratio 

 arms consisted of two equal coils, M, M, of manganin wire, 

 each of about the same resistance as the platinum coils, and 

 wound together on the same spool and immersed in oil. The 

 platinum coils had a resistance varying between 15 and 

 45 ohms in the various experiments. The glass tubes on 

 which the platinum coils were wound were placed in brass 

 tubes passing through a water-bath. When a specially 

 steady balance was required, the water-bath was completely 

 enclosed in a box and surrounded with lagging to reduce 

 the changes of temperature to a minimum. In most of the 

 experiments the correction for change of balance during 

 the time of a complete experiment was small and easily 

 allowed for. By means of an adjustable resistance in parallel 

 with one of the coils, a nearly exact balance was readily 

 obtained. A Siemens and Halske moving-coil galvanometer 

 was employed of resistance 100 ohms. This had the sen- 

 sibility required, and was found to be very steady and proved 

 in every way suitable. 



