Changes in Resistance hy a Valve Met/tod. 2 ( J5 



(2) With the same cell, and a potential of —2*3 volts 

 applied to the grid, the circuit functioned at the transition 

 point T lf where a change of resistance causes discontinuous 

 changes in the frequency. By tuning the standard circuit 

 to unison, a marked change in the number of bents per second 

 caused by the two telephone notes could be produced by a 

 candle at sixty feet. 



(3) Similar results were obtained with a "Thalofide" cell. 



Continuous change in frequency caused hy change in 

 resistance. 



A convenient method of expressing the sensitiveness to 



change in resistance is to define the sensitiveness as d/-r--rr 



or R..— /r. Thus, along the continuous portions of the curves 



in fig. 2, for a given R the greatest sensitiveness occurs on 



the curve of the greatest slope; while along any straight 



part of a curve the sensitiveness increases with the resistance. 



Increase in C causes the curves to slope more steeply. In 



fig. 2, curve (a) , ^ = : — . Hence, for a resistance of 5 X 10 4 <w, 



the sensitiveness = ——- — =2500 — i. c, if the two circuits 



be tuned to unison, one beat per second will be produced 

 when the resistance changes by one part in 2500. Curve (a) 

 lias not been worked out beyond /=3000, but the note con- 

 tinues to rise as R increases, and if the curve remains a 

 straight line, a sensitiveness of 10,000 should be obtained 

 with the selenium cell of resistance 2 x 10 4 <w. 



Comparison of capacities. 

 From fig. 3, curves (a), (Y), (/), it is evident that an 

 unknown capacity can readily be determined in terms of a 

 known one by obtaining a transition point The method is 

 as applicable to condensers of a few cms. capacity as to those 

 of the order of 1 m.f. 



Further investigations. 

 A number of interesting phenomena take place in the 

 neighbourhood of the transition points — such as, for example, 

 the discontinuities in frequency which occur near I\. These 

 phenomena are under investigation, and a theoretical dis- 

 cussion of the results is reserved till further experimental 

 results have been obtained. 



