322 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The actual arrangements in making this cahbration for the tempera- 

 ture coefficient of the pressure coefficient of the coil were as follows. 

 The upper cylinder of the hydraulic press in which pressure was 

 produced contained in addition to the moving plunger a steel shell 

 in which was as large a quantity of mercury as convenient, about 

 150 gm. This upper cylinder as well as the entire lower part of the 

 press was surrounded by a tank containing ice and water, by which 

 the temperature of the mercury could be kept continuously and 

 accurately at 0°. A heavy nickel steel tube led out of the lower end 

 of the upper cylinder through the bottom of the tank, and connected 

 with the lower cylinder in which was the manganin coil under exami- 

 nation. This lower cylinder was placed in an oil bath with thermo- 

 static regulation, by which the temperature could be set at and 

 retained at any desired value. The experimental procedure was as 

 follows. The temperature of the lower bath was set at any desired 

 value, and the pressure increased until the freezing point of mercury 

 at 0° was slightly passed. The mercury then froze with decrease of 

 volume, thus bringing the pressure back to the known equilibrium 

 value at 0°. After equilibrium had been reached, the resistance of 

 the manganin coil was read. The pressure was then lowered slightly 

 by withdrawing the piston. This was followed by automatic restora- 

 tion of the equilibrium pressure, brought about by melting of the 

 frozen mercury with increase of volume. The transition point was 

 always so sharp that no difference could be detected in the equilibrium 

 pressure whether approached from above or below. The temperature 

 in the lower cylinder containing the manganin was then changed to 

 another desired value. This change of temperature, if it were an 

 increase, would naturally carry with it a rise of pressure, but the 

 pressure is then automatically lowered by the freezing of the mercury. 

 After a steady state is reached, the new value of the manganin re- 

 sistance is read, and then the pressure lowered again by slightly 

 withdrawing the piston, and the value of the resistance noted again 

 after the equilibrium conditions have been restored from below. 

 In this way the coil can be calibrated over the entire temperature 

 range contemplated for the experiments. Of course this calibration 

 is good only for one fixed pressure, but in view of the proved linearity 

 of the pressure-resistance relation within 1/10% from 0° to 50°, it 

 seemed safe to let the calibration go at this one determination, particu- 

 larly since no eifect could be found. 



The calibration of the manganin was carried out at five tempera- 

 tures; 25°, 45°, 65°, 85° and 110°. No appreciable change of the 



