274 BRIDGMAN. 



perature by thermostats. Through the tube connecting the cyhnders, 

 and insulated from it, runs the wire on which measurements are to be 

 made. This is metaUically connected to the inside of the upper cyl- 

 inder, but passes out through an insulating plug at the bottom of the 

 lower cylinder. The circuit is completed outside the cylinders by a 

 wire of the same metal, making connection through the insulating 

 plug at the lower end, and at the upper end making a screw contact 

 with the steel of the upper cylinder. The return wire is broken by 

 the insertion of copper leads connecting with the galvanometer and 

 the potentiometer for measuring the electro-motive force. The 

 terminals of the copper leads are connected to the return wire inside 

 a constant temperature bath so that no e.m.f. (or at least no variable 

 e.m.f.) is introduced here. The two cylinders and the connecting 

 pipe are filled with liquid to which pressure may be applied. 



The circuit consists of the metal AB about 18" long under a constant 

 hydrostatic pressure, between the two ends of which there is a known 

 temperature difference equal to the difference of temperature of the 

 two thermostated baths; a length of nickel steel BC which forms the 

 stem of the insulating plug, in which there is a stress gradient from the 

 whole hydrostatic pressure -inside to zero at the outer end, but in which 

 there is no temperature gradient; a length CD about 4 ft. long of 

 the metal under examination, running at atmospheric pressure from 

 the temperature of the lower bath to that of the upper bath; the copper 

 part of the circuit from D to E, entering and leaving at the same 

 temperature; a short length, about 6 inches, of the metal under 

 examination running from the copper at E to mechanical contact 

 with the steel of the cylinder at F; and finally the circuit is completed 

 through the solid steel of the cylinder between F and A, in which there 

 is an intense stress gradient, but no temperature gradient. It is 

 evident that the e.m.f. of this circuit is merely that of the uncom- 

 pressed metal against the compressed metal between the temperatures 

 of the two baths. 



The measurements consist in finding the e.m.f. in circuits of 

 different metals under different temperature differences and different 

 pressures. Of course, if the inside and outside wires are precisely 

 alike, there is no e.m.f. for any temperature difference unless there is 

 also a pressure difference. But this ideal condition was seldom 

 attained, as the wires were never precisely alike; the readings had to 

 be corrected by the zero reading, which of course varied with the 

 temperature difference. 



The two cylinders were connected to the apparatus for producing 

 pressure through a tube screwed into the upper end of the top cylinder. 



