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PROCEEDINGS OF THE AMERICAN ACADEMY. 



the mica next the stem, where it is unsupported by the steel at the 

 rear surface. G is an insulating tube of glass. It is well to secure 

 the steel piece B against working 

 loose by the nut and hard rubber 

 washer at A. This plug is the most 

 permanent so far found ; one has 

 been subjected to 6500 kgm. up- 

 ward of seventy times with no sign 



W 



Figure 3. Preliminary form 

 of insulating plug for higher 

 pressures. M, mica washers; 

 G, marine glue to prevent leak. 

 Eventually the glue is forced 

 by the pressure past the mica 

 washers. 



A 



Figure 4. Final form of insulating 

 plug. M, mica washers; R, soft rubber 

 to prevent leak; S, steel washer to pre- 

 vent leak of the rubber past the mica ; 

 G, insulating tube of glass; A, nut to 

 keep the steel stem and the enlargement 

 B from working loose. 



of leak. The insulation resistance of these plugs is high enough for the 

 work in hand. Initially it is over 10 meg-ohms. With successive 

 applications of pressure the resistance drops considerably, finally 



