( 'hanges in Resistance by a Valve Method. 



2ys 



With farther decrease in C the silent region spreads 

 towards the right. Curve {e) shows a stage in its deve- 

 lopment. When = it extends from R = 4*8 x 10 4 a> to 

 R = infinity. 



At this stage it is evident that we have here a method of 

 measuring resistance or changes in resistance. Firstly, 

 using both circuits, we may insert the unknown resistance 

 in R, and alter R and C till a suitable frequency is obtained 

 in unison with the standard circuit. If, then, the unknown 

 resistance be removed and R increased till the same 

 frequency is again obtained, the required value can be 

 found. Secondly, using the experimental circuit only, R 

 may be varied till the sound in the telephone stops or starts — 

 i. e., till a border of a silent region is reached. These 

 transition points are well defined, and precise measurements 

 can be obtained. In fig. 2 (e), T x is the first transition point, 

 T 2 the second, the region between them being silent. 



The simplicity of the second method leads to inquiry into 

 the possibility of displacing the transition points to positions 

 suitable for dealing with resistances of any magnitude. 



Fig. 3 shows the value of R required to reach a transition 

 point when changes are made in K, 0, or Yg (potential applied 



Fig. 3. 



fa) (f) 



10x10 



20xlO J 



30X10 1 



40x10' 



to grid). On passing, in the direction of increasing R, across 

 any of the curves which meet at A, we pass from sound to 

 silence. In the case of curves (e) and (/) we pass from 

 silence to sound. In other words, the curves (a), (b), (c), (d) 

 are loci of the first transition point T\, while (e) and (/) refer 

 to T 2 . Each curve is labelled with the variable under con- 

 sideration, the other variables being maintained at zero, 

 except in the case of curve (/). 



Movements of the first transition point T\. 

 This point may be moved indefinitely to the left by in- 

 creasing the main capacity K [curve (a)], or to the right 



