HIGH PRESSURE BRIDGMAIST. 201 



Finally, bone was found to be satisfactory from both points of 

 view. 



Precautions relating to the freedom of constraint from the vis- 

 cosity of the transmitting medium, the purity of the substances 

 examined, the accuracy of the temperature determinations, and the 

 seasoning- of coils by exposures to high pressures and various tem- 

 peratures were carefully attended to. Measurements of the change 

 of resistance were made at intervals of 1,000 kilograms at 0°, 25°, 

 50°, 75°, and 100°. Two readings were made at the maximum and 

 two zero readings, one before and one after the run. There is 

 usually no perceptible " hysteresis " or difference between the read- 

 ings on increasing pressure and those found later on decreasing 

 pressure. I had not expected results so favorable. 



After every change of pressure some time is necessary before the 

 next reading can be made because of temperature disturbance due 

 to the heat of compression. This change of temperature is in many 

 cases so great as to entirely mask the effect of change of pressure. 

 The effect is very troublesome, as it may need as much as 30 or 45 

 minutes to reach temperature equilibrium for each change of pres- 

 sure. iWithout some trick of procedure, a run at a single tempera- 

 ture might occupy seven or eight hours and is excessively tedious. 

 This was avoided by running somewhat beyond the pressure desired 

 and then, after most of the heat of compression had been dissipated, 

 bringing the pressure back to the desired mark. With a little 

 practice temperature equilibrium is reached in five or seven minutes. 

 If the apparatus is in good running order a complete run on one 

 substance at one temperature could usually be made in about two 

 hours, and, including all manipulations, runs at two different tem- 

 peratures could easily be made on a single substance in a working 

 day. 



GENERAL CHARACTER OF RESULTS. 



The effect of pressure on all the metals tried, with the exception 

 of antimony and bismuth, is to decrease the resistance. To a first 

 approximation the relation between pressure and resistance is linear. 

 To a second approximation the relation is not linear, but the initial 

 rate of decrease of resistance is in all cases greater than that at 

 higher pressures. 



In the case of some of the softer metals, unusual means were taken 

 both in the preparation of the wire and in its use, which may be 

 of interest to the reader. As, for example, consider indium. A 

 sample of only 1 gram in amount was available. This metal is as 

 soft or softer than lead. It was extruded — that is, forced out — 

 into a wire of 0.006 inch diameter with a die of special construction. 

 Indium oxidizes much less rapidly than lead; after extrusion the 



