﻿214 Mr. J. A. Brown on the Potential required 



Procedure. 



The method of procedure was briefly this:— The pressure 

 having been adjusted and carefully read on the gauge, the 

 current w T as put on and readings of the galvanometer and 

 voltmeter taken as soon as possible. Then the resistance was 

 decreased and a new value taken. The largest current having 

 been attained, the reverse process was followed, the resistance 

 increased and various new points on the curve tor the current 

 determined. In this way the effect of any permanent change 

 in the pressure could be at once detected. The curves of 

 fig. 2 show the method of procedure. 



Fig. 



330 



320 



310 



300 



290 



280 



£70 





Zn. 













^^C 





















/ 



<f 















// 



/ 





jo?~es 



sure = 



0-76 mm 





; 



& / 

















'ct 















:0 30 40 50 



(x 1-I0~ S = Ampere .) 



60 



70 I 



The first reading obtained was the one marked a on the 

 diagram. The resistance was then gradually decreased until c 

 was reached, and then the reverse process carried on down to b. 

 The difference in the potentials of a and h was so small that 

 taking the average of these two curves would introduce no 

 great error ; and this was done, the pressure being assumed to 

 be the mean of the readings on the gauge before and after 

 the series. (But see later, the explanation given for the form 

 of the curves below the critical pressure.) 



