1920.] Measurements with Quadrant Electrometers. 19 
ever possible, and as shown in another paper,! sodium used as a 
drying agent tends to increase rather than diminish the natural 
leak of the insulator which it is intended to protect. After 
trying numerous modifications of this type of insulator, using 
amber, sulphur or paraffin for the insulating material, with 
one of these have proved satisfactory in practice for work of 
the highest degree of accuracy, either as regards the degree of 
insulation secured or the cons taney of theleak. This may be 
seen from the following measurements, typical of a large 
number of similar observations on ihe behaviour of insulators 
protected as shown in Fig. 2. These insulators were of exactly 
the same size except that the surface of the paraffin was not 
grooved. 
1 Utility of desiccants in electrostatic work. 
