PRESSURE ON RESISTANCE OF METALS. 581 



pressure in order to avoid any error dne to the direction of change of 

 pressure. These readings were made alternately; at 0, 1000,. 2000, 

 4000, GOOO, 8000, 10000, 12000, with increasing pressure, and at 12000, 

 11000, 9000, 7000, 5000, 3000, and with decreasing pressure. There 

 is no perceptible hysteresis. Figure 2 for lead shows this. This entire 

 absence of hysteresis was very gratifying; I had not expected results 

 so favorable. At the two lower temperatures of the earlier runs a 

 small effect in a direction opposite to that of hystersis was sometimes 

 found. This was traced to the viscosity of the kerosene transmitting 

 pressure; it entirely disappeared on using the less viscous petroleum 

 ether to transmit pressure at 0° and 25°. 



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; immedi- 



Pressure 



Figure 2. The deviation from linearity for lead against pressure, both on 

 an arbitrary scale. The purpose of the diagram is to show the entire absence 

 of hysteresis; the circles show the measurements with increasing pressure, and 

 the crosses with decreasing pressure. One small division corresponds to 3% 

 of the total change of resistance produced by the maximum pressure. 



ately after changing pressure the resistance of most substances changes 

 in a direction opposite to that of the final change. Some substances, 

 like lead, in which the ratio of pressure coefficient to temperature 

 coefficient is high, do not show the initial reversal, but in most cases 

 the immediate change may be 5 or 10 times as great as the final change 

 and in the opposite direction. This effect is very troublesome, as it 

 may need as much as 30 or 45 minutes to reach temperature equili- 

 brium after each change of pressure. Without some trick of procedure 

 a run at a single temperature might occupy seven or eight hours and is 

 excessively tedious. The time to reach equilibrium may be very 

 much shortened by running the pressure beyond the desired final mark 

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

 bringing the pressure back to the desired mark. The heating effect 

 during this second change of pressure is opposite in direction from the 



