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A STANDARD CONDENSER OF SMALL CAPACITY. 
By R. R. Ramsey. 
In radioactive measurements of substances which are very feebly radio- 
active it 1s necessary to have an electroscope which is very sensitive. One 
of the conditions to obtain this result is, the electroscope must have a very 
small capacity. A capacity of one to ten centimeters. A sphere has a capac- 
ity equal to its radius when far removed from other objects but when brought 
near to the electroscope its capacity changes to a value which depends upon 
the position, size and shape of the electroscope. 
It is customary to use a cylindrical condenser. The capacity of a cylindri- 
cal condenser is 
L 
C= 
2 loge Ri/Re 
where C is the capacity; L is the length; R; is the inside radius of the out- 
side cylinder; R, is the radius of the inside cylinder. This formula gives the 
capacity if the effect of the ends can be neglected. This requires that the 
length should be great compared to the difference of the two radii. When 
these conditions are met the capacity will be 100 em. or more. 
In order to correct for the end effects I have made a condenser in three 
sections, the construction of which is illustrated in the cross sectional draw- 
ing. The middle cylinder is made of a brass rod about 9 millimeters in diam- 
eter. The outside cylinder is made of brass tubing whose inside diameter is 
about 3.6 em. The diameters are chosen large in order that the accuracy of 
measurement may be great. The ratio of the diameters is made large in order 
that the capacity per unit length may be small. 
The length of the end sections is 10 em. The length of the middle sec- 
tion is 20 em. The middle rod is held in place in the end sections by means of 
sulphur. This was accomplished by means of two wooden dises which were 
accurately turned to fit in the ends of the large cylinder and hold the middle 
rod in the center. These dises were placed in the ends of the end sections. 
The end section was stood upon the outside end and melted sulphur was 
poured through a hole in the top dise until the cylinder was about one-third 
filled. The dises were removed after the sulphur had hardened. Dowel pins 
are placed on the middle rod to hold the middle section in place. 
