264 Miss P. G. Fawcett. The Electric Strength of [June 21, 



the opposed faces of the plates, it would take a longer path, and pass 

 between the connecting wires on the backs of the plates. When the 

 distance was 0'047 in., the discharge began to pass between the backs 

 of the plates when the pressure was reduced to about 2 mm. 



In the later experiments this was prevented by using plates em- 

 bedded in ebonite discs, only leaving exposed the faces between which 

 the spark was intended to pass, and by making the connexions with 

 indiarnbber covered wires. Even then, after the plates had been 

 used for some time, the indiarubber showed signs of giving way, and 

 a discharge occasionally passed partly between the wires and partly 

 between the plates. 



The plates were kept at the right distance apart by placing between 

 them small flat pieces of ebonite of the same thickness (O047 in.). 

 In the earlier experiments, the plates were in an ordinary bell- jar 

 standing on a flat surface, the rim being greased with a mixture of 

 bee's-wax and vaseline. Thinking it possible that there might be 

 some vapour given off by the grease, I arranged the apparatus so that 

 it could be made air-tight without grease. For this purpose I used a 

 rather narrow bell-jar, closed at the bottom by an indiarubber stop- 

 per, through which passed three glass tubes for conveying the con- 

 necting wires, and for communicating with the air-pump and the gas- 

 holder. The jar, with its stopper, was placed in a vessel containing 

 mercury, so that the junction of the glass and indiarubber was im- 

 mersed, the tubes being bent so that their ends came above the 

 mercury. The arrangement is shown in the accompanying figure ; 

 a, a are the ebonite discs in which the plates are embedded ; the wires, 



Hattery 



b, 6, pass through the ebonite, and are covered with indiarubber 

 throughout their length until they come out into the open air at the 

 ends, A, A, of the tubes, c, c, which are sealed with sealing-wax. 



