BRIDGMAN. — A SECONDARY MERCURY RESISTANCE GAUGE. 227 



mercury resistance was therefore about ^ho ampere. It was neces- 

 sary that the current be about as small as this to avoid heating effects 

 in the very fine mercury thread. With this low current, however, the 

 key might be closed indefinitely, with no apparent change in the re- 

 sistance of the mercury. 



In carrying out the measurements, the first and most considerable 

 difficulty that presented itself was the designing of a suitable insulating 

 plug for leading the electrical connections into 

 the pressure chamber. Amagat, and most 

 investigators following him, have used as in- 

 sulating plug a cone of steel (B, Figure 2) 

 separated from the surrounding walls of the 

 pressure chamber by a thin layer (A) of hard 

 rubber or ivory. Any such arrangement as 

 this proved unsuitable for the pressures dealt 

 with here, the hard rubber flowing completely 

 out of the conical crevice, and exuding in the 

 form of a more or less continuous cylindrical 

 tube. Various modifications of this, using 

 the tougher red fibre instead of hard rubber, 

 were tried with little success. Silk also was 



/ 



used as an insulating material and with bet- 

 in 



Figure 2. Amagat's 

 insulating plug. A, insu- 

 lating shell of hard rubber 

 or ivory; B, cone of steel. 

 At high pressures the insu- 

 lating material, A, flows out 

 of the crack. 



ter success. The silk was cut out in the 

 form of a number of discs and placed around 

 the shank of the cone, which was then forced 

 into its seat. It was found advisable to make 

 the cone and its shank from one piece of 

 steel, otherwise they were pulled apart by 

 the friction of the silk. This form of plug has a high enough insu- 

 lating resistance and is tight, but has the disadvantage of not being 

 permanent. After ten or twenty applications of pressure the silk 

 loses all semblance of structure, and leaks more and more rapidly 

 with every successive application of pressure. 



The cone was now given up and mica tried for insulation, tightness 

 being secured by a layer of marine glue (G, Figure 3). The mica 

 showed no tendency to flow or crumble at the unsupported edge at A. 

 This device was much better than the silk, but it too was not perma- 

 nent, the marine glue being eventually forced past the mica washers 

 which were a drive fit in the hole. In the form finally adopted 

 (Figure 4) the mica insulation is kept, but tightness was secured by 

 a layer of soft rubber, R, between the mica washers, M. The small 

 steel washer S was necessary to prevent the rubber forcing its way past 



