ELECTRICAL RESISTANCE UNDER PRESSURE. 63 



Experimental Method. 



The metals measured in the pre\'ious investigation could all be made 

 into wire of small diameter, and therefore high resistance, and nearly 

 all of them could be soldered. The resistance was measured on a 

 Carey Foster bridge. It was essential to accuracy that the resistance 

 be so high and the contacts so good that the relative changes in contact 

 resistance under pressure were negligible. Very few of the metals of 

 this investigation satisfy this condition. For the measurement of the 

 effect of pressure on the resistance of these, some sort of potentiometer 

 method is indicated. The resistance of the contacts then is without 

 effect, and relatively small changes on small resistances can be meas- 

 ured accurately. 



The application of a potentiometer method demands four leads to 

 the pressure chamber, two current leads, and two potential leads. 

 Three of these leads must be insulated from the pressure chamber; 

 the fourth may be grounded on the walls. An insulating plug carry- 

 ing three leads was used in this work. It is an obvious modification 

 of the single terminal plugs pre\'iously used. It may be mentioned 

 that the modification of the design of the plug suggested on page 641 

 of the paper referred to, making possible the use of the ring packing, 

 has been in use now for a number of j'ears, and was used with the new 

 three terminal plug. The chief difficulties encountered in the three 

 terminal plug were the mechanical difficulties of securing the accuracy 

 required in getting three terminals into a small space. The fine in- 

 sulated stems were made of piano w'ire 0.032 inches in diameter, held 

 into a head at the top by a special wedge grip. Each of these stems 

 with its head was tested before assembling with a tension of 270 

 pounds, corresponding to a pressure of over 20,000 kg/cm^. Without 

 a preliminary test there is danger that the stem will pull out of the 

 head under pressure. The mica washers used for insulation of the 

 plug were punchings, made a tight fit for the hole, which was 0.209 

 inches diameter. They were forced into place in the plug, and drilled 

 concentrically for the stem with suitable jigs after they were in final 

 position. Later the mica washers have been replaced with pipe-stone 

 washers with more satisfactory results. The chief difficulty en- 

 countered with this plug has been in using it near 100°. Here the 

 rubber insulation gives out under the action of the petroleum ether 

 with which pressure is transmitted, and the insulation has to be re- 

 newed much more frequently than with the larger plug. 



