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PROCEEDINGS OF THE AMERICAN ACADEMY. 



leak of any kind. It has not been tested higher, but there is no reason 

 why it should not hold until the steel cylinder stretches enough to 

 leak. It is a satisfaction in using this connection to feel that this part 

 of the apparatus is at least no weaker than any other. 



The Pressure Gauge. 



An . 



Without doubt the most convenient means of measuring high 

 pressure is to measure the change produced by the pressure in the 

 electrical resistance of some such alloy as manganin. The details 

 have been given in a previous paper.^ The only structural difficulties 

 with the method are those incidental to getting 

 electrically insulated leads into the cylinder. The 

 principle of the insulating plug has also been 

 described in a previous paper, but not in much 

 detail, and since several improvements have been 

 introduced, it will pay to redescribe it. The plug 

 is shown in Figure 11. It consists essentially of 

 an outer shell, A, to which one electrode is silver 

 soldered, and running through it and insulated 

 from it, a thin steel stem, F, which forms the 

 other electrode. The outer shell of the plug, A, 

 is made of Ivrupp Chrome Nickel steel, hardened 

 in oil. The packing of the shell used at present 

 is soft rubber, after the same design as that used 

 for the tubing at lower pressures. This packing 

 is not shown in the diagram. There is no weak- 

 ening of the plug here by a thread on the outside, 

 so that the "pinching-off " eflFect is not nearly so 

 troublesome as in the case of the packing for the 

 tubing. Moreover, by making the shell with a 

 taper shoulder, and placing over this a soft steel 

 cone, against which the rubber bears, thus provid- 

 ing for the possibility of slip of one metal part over 

 the other in the locality where the "pinching- 

 off" effect would be most likely to occur if the metal were of one solid 

 piece, and by using rubber washers only | of an inch thick, the danger 

 of "pinching-off" is reduced to a minimum. However, this design 



Figure U. The 

 insulating plug for 

 leading electrical 

 connections to the 

 interior of the pres- 

 sure chamber. The 

 scale of the diagram 

 is I actual size. 



3 P. W. Bridgman, These Proceedings, 47, 321-343 (1911). 



