316 
The capacity of the middle section is calculated by the formula. The 
electroscope is charged to a potential V;. The charge on the electroscope is 
divided with the condenser, all sections being used. 
If C, is the capacity of the electroscope. 
C, is the capacity of the end sections. 
C; is the capacity of the middle section. 
V, is the initial potential. 
V2 is the final potential. 
then since 
Q=CiVi=(Ci+C2+Cs3) Vo 
Vi/V2= (C; +C2+Cs) /Ci a 
The electroscope is again charged to a potential V’;. The charge is again 
divided with the condenser, the end sections being used. 
Then we have 
V'1/V’2 = (C, +C») {Gi —Ts 
combining the two equations involving r; and r2 we get 
3, =C3/ (4-12) 
In case that one has a steady ionization current as in the ease of radium 
emanation in an emanation electroscope after three or four hours, one can 
allow-the electroscope to discharge through a certain potential difference, dV, 
first with the electroscope alone, then with the ends of the condenser con- 
nected to the electroscope, and then with the entire condenser connected. 
Since i=C dV/t and dV is constant, we have, 
Ci /t1 = (Ci + C2) /te = (Ci + Co +Cs) /ts = C3/ (t-te) 
Care must be taken to see that the current is constant during the obser- 
vations. If the current is due to 8 or y rays there is danger of the 
air inside of the condenser being ionized and thus producing a variable current. 
The capacity of the middle section of the condenser which I have is 
8.06 em. The capacity of the end sections is found by experiment to be about 
17 em. Thus, since the combined length of the ends is the same as the middle 
section, the end effects plus the dielectric effect of the sulphur is about 9 em. 
Department of Physics, Indiana University, December 1, 1915. 
