BRIDGMAN. — HIGH PRESSURE EXPERIMENTING. 641 



does not entirely do away with the effect; it is safe to count on a 

 rupture about once a year if the apparatus is used continually to 12000 

 kgm. Failure of the plug need not result in complete rupture. Once 

 or twice I have been troubled with an obscure leak which I finally 

 traced to a crack at the base of the cone of A, where the " pinching-off " 

 effect might be expected to occur. If ever occasion should arise to 

 make new plugs, I intend to redesign them so as to use the ring pack- 

 ing of steel now used for the connecting tubes. 



The insulating properties of the plug are provided by two layers of 

 mica washers, B and C, turned (not punched) so as to be a force fit 

 for the hole. Tightness against leak is provided b}^ a la\'er of rubber, 

 D, between the two layers of mica. The fine stem, F, is insulated 

 from A for the rest of its length by a thin glass tube, G, slipping into 

 the annular space between F and A. A small cylinder of hard rubber, 

 H, at the outer end completes the insulation. The insulating proper- 

 ties of the plug are improved if all the parts are dipped into paraffine 

 heated to from 120° to 140° immediately before assembling, and if 

 they are kept hot enough during the assembling so that the paraffine 

 remains melted. The insulation resistance is at least 100 megohms, 

 the limit of the measuring instrument used. 



It will be noticed that the insulating packing uses again the prin- 

 ciple of the unsupported area, the central stem in this case being the 

 unsupported part. But the area of the stem is so slight that the excess 

 pressure provided by it may not be sufficient to overcome the friction 

 in the mica washers against the side of the plug. It is therefore neces- 

 sary to make the rubber washer initially considerably larger than the 

 hole, or else the tightness will not be permanent. A washer initially 

 Y2 oi an inch to go into a hole Ye of an inch in diameter is not too 

 large. 



The steel disc at E is necessary to prevent the rubber blowing out 

 past the mica washers along the stem. This disc should be of hard- 

 ened nickel steel. The stem F passing through the washers is also 

 of hardened nickel steel. It, together with the head at its upper end, 

 must be made from one piece. Attempts to braze or screw the head 

 onto a piece of wire have uniformly failed. This thin stem is also 

 likely to fail by the "pinching-off" effect, where it passes through the 

 rubber, or if the friction of the washers is too great, it may be torn 

 apart by the tension afforded by the expansion of the washers during 

 decreasing pressure. Several times I have observed rupture from 

 this cause at 6000 kgm. after decreasing pressure from a maximum of 

 12000. 



