64 BRIDGMAN. 



The method of electrical measurement is a null substitution method. 

 In series with the material under pressure is a second resistance, part 

 of which consists of a slide wire. By means of a variable slider, it is 

 possible to tap off a variable part of this second resistance. A throw- 

 over switch enables either the potential terminals of the pressure coil 

 or the variable part of the series resistance to be connected to the 

 indicating galvanometer. This galvanometer is a Leeds and Northrup 

 high sensitivity moving coil instrument. At the scale distance used 

 its sensitiveness was 10"^ volts; its sensitiveness could be decreased 

 with appropriate shunts. 



The measurements are made by adjusting the various resistances 

 so that there is no change of deflection on operating the throw-over 

 switch. The resistance of the pressure coil may then be computed 

 from the known values of the other resistances. In practise, sensi- 

 tiveness and speed of operation are increased by throwing into the 

 galvanometer circuit another e.m.f. approximately equal and opposite 

 to the potential difference across the pressure coil, so that the actual 

 deflection is approximately zero. This balancing e.m.f. should be 

 variable over a wide range and should be fairly constant. To produce 

 it, I used the apparatus previously used in measuring thermal e.m.f. 

 under pressure, tapping across the former pressure terminals. The 

 refinements of that apparatus were not necessary, but it was easier to 

 use apparatus already at hand than to construct new. The apparatus 

 by which the variable balancing e.m.f. was applied is indicated by 

 VE in Figure 1 . 



The details of the connections are shown in Figure 1. When ad- 

 justments are made we know that the potential drop around R (the 

 pressure coil, which is usually a small fraction of an ohm) is equal to 

 that about R3 plus ri. This latter drop of potential may be computed 

 from the known values of Ri, Ro, Rz, ri, and ro, and gives 



^ ^ 7?,(i?3 + n) 



Ri + R2 + i?3 + n + n 



The resistance n (and r^ accordingly) is the only resistance varied 

 during a pressure run; the other resistances Ri, R2, and Rz being 

 appropriately chosen and then kept constant during each run. Since 

 r\ -\- r2 = r { di constant) we have, for any one run, 



AR = C An 



or, putting R = Ro -{- AR, and writing rw for the initial value of ri 

 corresponding to R = Ro 



