Heating Effect of Radium and its Emanation. 323 



of the heating effect of the emanation and of the current 

 were made for a period of two hours. The emanation-tube 

 was then removed and the change of the balance in the 

 interval determined. The change of balance due to slight 

 alterations of the temperature of the room was usually found 

 to be quite regular and small, and could be easily corrected 

 for if necessary. 



In order to determine the heating effect, it was necessary 

 to measure accurately the amount of emanation at any 

 moment in the tube and the current! through the heating- 

 coil. The 7-ray effect of the emanation-tube at a definite 

 time was compared in terms of the Rutherford- Boltwoocl 

 standard' by the electroscopic method, and also by the balance 

 method developed by Rutherford and Chadwick. The 

 authors are indebted to Mr. Chadwick for his kind assistance 

 in these measurements. The results obtained by the two 

 methods were in good agreement. A correction of 0*3 per 

 cent, was made for the absorption of the y rays in the walls 

 of the emanation-tube. The heating effect of the emanation 

 was assumed to decrease exponentially with a half- value 

 period of 3*85 days. This period of decay was verified on 

 several occasions by direct measurement of the heating effect. 

 The current through the heating-coil was determined by 

 measurement of the E.M.F. of the accumulator by a care- 

 fully standardized voltmeter, and the total resistance of the 

 circuit. 



The measurements of the heating effect made with different 

 quantities of emanation were in close accord, and the mean 

 of each series of measurements agreed within 1 part in 500. 

 In this way it was found that the heating effect of a quantity 

 of emanation which gave the same 7-ray effect as one gram 

 of radium (Rutherford-Boltwood standard) was 95*95 + '05 

 gram calories per hour under the experimental conditions. 



It is necessary, however, to correct this value to obtain 

 the heating effect of one curie of emanation, i. e. of the 

 quantity of emanation in radioactive equilibrium with one 

 gram of radium. The amount of the products radium A, B,. 

 and C in transient equilibrium with the emanation are some- 

 what greater than the amounts in equilibrium with the same 

 quantity of emanation which is maintained constant. This 

 point has been discussed by Moseley and Makower *, and 

 by Rutherford and Chadwick f. The amount of radium B 

 is 0*54 per cent, and of radium G 0'89 per cent, in excess of 

 the true equilibrium amount. Moseley and Makower showed 



* Moseley and Makower, Phil. Mag. xxiii. p, 302 (1912). 



t Loc. cit 



