1046 Number of «- Particles emitted by Thorium. C. 



equilibrium with 1 mg. of radium is 3'72x 10 7 per second, 

 with a probable error of *54 per cent. But we have just 

 seen that this amount of radium C gives only 91 per cent, of 

 the total ionization produced by radium and its products. 

 Therefore the rate of emission of a-particles by the amount 

 of radium which will give the same ionization as 1 mg. of 

 radium when measured through 3*3 mm. of lead is 3'72x 



Iqt x 122 =4-09 x 10 7 per sec. Consequently, thorium C of 



equal 7-activity emits at the rate of 4'09 X 10 7 X -75 = 3-07 x 

 10 7 per second. This number involves the error of 1*7 per 

 cent, in the ratio *75 ; the error of '54 per cent, in the deter- 

 mination of 3'72 x 10 7 ; the electroscope error of '8 per cent, 

 acrain entering twice in the absorption measurements ; and 



Fig. 4. 



x/O^/oer seconcr. 



















V^ 



























































































































































Mms. 



Leaaf. 





















3-3 4-3 6-3 8-3 10-3 12-3 M-3 



Number of 8*6 cm. ^-Particles emitted by ThC of y- Activity 

 1 mg. Radium. 



the error due to the weighing of the lead sheets and varia- 

 tions in thickness of those sheets. This last error can only 

 be estimated, but it should not be greater than 1*5 per cent. 

 Combining all these factors gives a probable error of 2*5 

 per cent. 



The amount of thorium C which will give the same 

 amount of ionization in an electroscope as 1 mg. of radium 

 and its products will vary with the thickness of lead through 

 which the 7-ray measurements are made. In fig. 2, curves 

 I. and III. are the absorption curves for thorium C and for 

 a radium standard, and curve VI. is plotted from the ratios 

 of the ordinates of curve III. to curve I. Starting with equal 

 7-activities of thorium C and radium at 3*3 mm. of lead, this 

 curve VI. evidently gives the amounts of thorium C which 

 will give at greater thicknesses of lead 7-activities equal to 



