BRIDGMAN. — MERCURY TINDER PRESSURE. 401 



single stroke. This upper cylinder was freely exposed to the air of the 

 room, with no special device to keep the temperature at any definite 

 value. 



The lower cylinder contained the mercury, the subject of the experi- 

 ment, and was surrounded by a bath kept at constant temperature by 

 a thermostat. The mercury was contained in a cylindrical steel shell 

 fitting loosely the hole in the cylinder. The same kerosene which 

 entirely filled all other parts of the apparatus and transmitted pressure 

 from the upper to the lower cylinder also entirely surrounded the 

 walls of the shell containing the mercury. This arrangement was 

 necessary, because if the mercury had been allowed to come in contact 

 with the walls of the cylinder, the steel would have become amalga- 

 mated, and the cylinder speedily ruptured. Whereas, the steel shell 

 in which the mercury was contained being surrounded on the outside 

 by kerosene, and so being subjected to a purely hydrostatic pressure 

 equal all over, experienced no amalgamation whatever. ^^ 



Screwed into the bottom of this lower cylinder, directly immersed in 

 the kerosene and so subjected to the same pressure as the mercury, 

 was an insulating plug carrying a coil of manganin wire with which the 

 pressure was measured. 



The procedure was as follows : After filling the apparatus with 

 kerosene, the thermostat was raised around it and adjusted to the 

 temperature desired. When a steady state was reached, the resistance 

 of the manganin coil was read on the Carey Foster bridge, thus giving 

 the pressure zero. The piston was then introduced into the upper 

 cylinder, and pressure increased to a value a few hundred atmospheres 

 short of the freezing pressure at the temperature of the thermostat. 

 After pressure equilibrium had been attained by dissipation of the 

 heat of compression, the pressure was read from the resistance of the 

 manganin coil and the position of the piston was read with the microm- 

 eter. Two readings of the resistance of the manganin were made 

 corresponding to the two positions of the eight-point switch of the 

 Carey Foster bridge. These readings were made before and after read- 

 ing the position of the piston. Since the circuits were non-inductive, 

 the bridge could be used with the galvanometer circuit permanently 

 closed. These two readings are, therefore, really two independent 

 readings of the pressure, and agreement between them is indicative of 

 the attainment of pressure equilibrium. It was not necessary to take 

 any such precautions in measuring the position of the piston, for the 



*^ See These Proceedings, No. 14, 1911, for a discussion of the amalgama- 

 tion of steel under high pressures. 

 VOL. XLVII. — 26 



