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The capacity of the middle section is calculated by the formula. The 

 electroscope is charged to a potential V,. The charge on ilie electroscope is 

 divided with I lie condenser, all sections being used. 

 If Ci is the capacity of the electroscope. 

 C2 is the capacity of the end sections. 

 C 3 is the capacity of the middle section. 

 Vi is the initial potential. 

 V 2 is the final potential, 

 then since 



Q = CiVi = (Ci+C2+C,)V, 

 V 1 /V 2 = (C 1 +C 2 +C 3 )/C 1 =r 1 

 The electroscope is again charged to a potential V'i. The charge is again 

 divided with the condenser, the end sections being used. 

 Then we have 



VVV' 2 = (C 1 +C 2 )/Ci=r 2 

 combining the two equations involving r x and r 2 we get 

 Ci=C,/(n-rO 

 In case that one has a steady ionization current as in the case 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/ti = (C, +C 2 )/t 2 = (Ci+.C,+C,)/t, = C 3 /(t 3 -t 2 ) 

 Care must be taken to see that the current is constant during the obser- 

 vations. If the current is due to p 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 cm. The capacity of the end sections is found by experiment to be about 

 17 cm. 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 cm. 

 Department of Physics, Indiana University, December 1, 1915. 



