the Radiation from Radioactive Bodies. 54:7 
The agreement between theory and experiment in Kohl- 
rauselr's paper seems to me to be of special interest, for the 
method used by him differs from the method employed in 
this paper, while the intensity of the radiation in his experi- 
ments was nearly twenty times greater than the strongest 
used in mine. 
I have to thank Professor Rutherford for the kind interest 
he has taken during the progress of this research. 
Physical Laboratory, 
University of Manchester. 
Jfote added March 12, 1908. — Since the above was com- 
municated a paper has been published by E. Meyer and E. 
Regener in the Verhandlungen der deutsclien physikalischen 
GesellscJiaft, No. 1, 1908. The authors also find, using a dif- 
ferent method to the writer, that the error increases with the 
square root of the intensity of the radiation. Further, they 
state that by measuring the error e and the saturation current 
i the charge of an ion may be determined. But the calcula- 
tion involves the number of ions produced by an a, particle, 
and this number was determined by Rutherford under the 
assumption that the charge of an a particle is identical with 
the charge of an ion. This, however, is still an unsettled 
question. 
I may add here that the number of a particles emitted per 
sec. from a given substance can be determined directly by 
simply measuring the error e and the saturation current i. 
For the error e in E.S.U. is given by 
e=±T$e<s/Z, 
where X is the number of ions produced by one a particle, e 
the charge of an ion, and Z the number of particles emitted 
per sec, while the saturation current is given in E.S.U. by 
i = XeZ. 
By division we get Z as function of e and i only. The agree- 
ment between the errors determined by theory and by 
experiment indicates that the calculated number of a particles 
emitted per sec. from a radioactive body of known activity is 
of the right order of magnitude. 
