460 
small. Therefore we wish a sensitive electroscope of small capacity. The 
cylindrical chamber is in reality a cylindrical condenser. Therefore we want 
the diameter of the rod small compared to the diameter of the cylinder, also 
the collar holding the insulating material should be short and have a large 
diameter compared to the rod. In short the dimensions of the insulated 
system should be as small as rigidity and other considerations will permit. 
To make the electroscope sensitive the ‘‘gold leaf’? should be very 
light and narrow. With Dutch leaf a strip between one and two millimeters 
wide will give a change of one mm. for a change of potential of five volts. By 
P L 
using a reading microscope with graduated eye piece a fraction of a volt can 
be detected. The electroscope should be made of brass tubing the thickness 
of whose walls is one mm. or more. Where such material is not to be had 
sheet tin can be used and will give satisfactory results, especially if used in 
a laboratory free from penetrating radiation. I will describe one which I 
made at a cost of a few cents and have found to give consistent results when 
compared with one made in Germany which cost $50. 
The cylinder A, Fig. 2, is a 1 lb. coffee can. B, is a tin can made in 
a local tin shop, 3x3x5 inches with a lid at the top. The lid of the coffee can 
and the lid of the rectangular can are soldered together and a short cylinder 
