184 electric units of charge^ — by g. c. laurence, b. sc. 



Eliminating End Effects by Using a Double Condenser 



The use of guard rings was obviated by making a double 

 condenser consisting of a long and a short cylinder of equal 

 diameter inside the one outer cylinder. (Fig. 2). By taking 



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the difference between the capacities of the two inner cylinders 

 end effects were satisfactorily eliminated. The cylinders were 

 mounted so that they could be rotated about the axis x, end for 

 end, thus putting the exposed end of the cylinder whose capa- 

 city is being measured in the same position as that previously 

 occupied by the other cylinder, thus eliminating errors due to 

 external electrostatic fields. While the capacity of one inner 

 cylinder was being measured in electromagnetic units the other 

 two cylinders were earthed. The wire hooks h, protruding 

 from the ends of the inner cylinders served in making connec- 

 tions to them. The difference in capacity in electrostatic units 

 was calculated from the expression C={L-S)/(2 log R/r), 

 where L and S are the lengths of the inner cylinders and r their 

 radius, and R the radius of the outer cylinder. 



