the Radiation from Radioactive Bodies. 545> 
by one a particle. Therefore the error measured in the ease 
of j3 particles, on the same electrometer and under the above- 
mentioned conditions, should be about one-fifth of that 
observed for a particles giving the same intensity of ioniza- 
tion. The smallness of the irregularities in Curves III. and 
IV. (fig. 4) compared with those shown in Curves I. and II. 
is thus to be expected. 
A special series of measurements has been made to show 
how the average error depends upon the intensity of the 
radiation. At the points R and U (fig. 3) two wires were 
placed which were made intensely active by exposure to the 
radium emanation. While the activity was decaying, the 
oscillations of the needle were observed at intervals, and 
curves were drawn showing the oscillations as accurately as 
possible over intervals of 5^ minutes. From these curves 
the average error was determined by counting the divisions 
passed over by the electrometer needle during the attempted 
balance, and dividing that number by the number of swings 
observed during the same time. The average error was also 
calculated theoretically. An example may be given. 
Experimental Determination of the Error. 
The data are taken from Curve II. (fig. 4). The number 
of divisions passed over in 5^ minutes, counting the maximum 
divergence both positive and negative, was 380 and the 
number of oscillations during that time was 64. Hence the 
average magnitude of one oscillation was -^- = 5*9 divisions, 
and the average deviation or the error ±2*95 divisions. 
Theoretical Determination of the Error. 
The error is given by \/Z where Z is the number of atoms 
breaking up during the time r. This number gives also the 
number of ex particles shot into the vessel during the time t. 
The time r is in this case the average time of swing, being 
5*8 sec. as an average taken from all the curves. The intensity 
of the radiation of one wire was 1100 divisions per minute, 
and therefore, as one division corresponds to the ionization 
produced by about 22 a particles, the number of particles 
shot into the vessel is 2*4 x 10 4 per minute, and therefore the 
number shot into both vessels in 5*8 sec. is 
2x2-4xl0 4 x||==4-7xl0 3 . 
The square root of this is 68. The average error taken over 
