IOWA ACADEMY OF SCIENCE 107 



distance between the keys k3 and k4 was varied. Between .01 and 

 about .12 sec. the linear relation required by equation '^x=Cd holds, 



At 

 but from .12 sec. to .4 sec. there is a slight tendency for the deflection 

 to grow less as the interval is increased. This slightly downward bend- 

 ing of the time-deflection curve for longer intervals of time probably 

 arises from the damping action of the closed coil moving in a magnetic 

 field. It was quite definitely settled that the linear relation between d 

 and At was obeyed for values of At as small as .0075 sec, but for values 

 less than this there was considerable doubt. It may be stated that the 

 method suggested for determining changes of resistance is applicable 

 through the use of equation Ax=Cd for quite large ranges of values 



At 



for Ax and At. The range of applicability will vary to some extent with 

 the apparatus used. There will enter such factors as the theory of 

 the galvanometer, the value of the resistance in the circuit, and self- 

 induction. 



In case the change of resistance Ax is large compared with the initial 

 resistance equation Ax=Cd can not be applied and the equation in- 



At 

 volving the resistances in the four arms of the bridge is not easily ap- 

 plied. The change of resistance is a function of the deflection regard- 

 less of how large it may be, but it is not easily computed. The easiest 

 way out of the difficulty is to determine the deflections obtained by the 

 unknown variable resistance for different intervals of time and then 

 to substitute various known resistances which will give a range of de- 

 flection covering those obtained by the unknown. The deflections with 

 the known resistances can be plotted to give a calibration curve. Then 

 the value of the unknown resistances, or the change of resistance, can 

 be read from the calibration curve. A simple illustration will explain 

 this application of the method, 



A Giltay selenium cell was connected at X in figure I. The selenium 

 cell was placed in a dark box in which was also a tungsten lamp. By 

 closing both keys kl and k2 the storage battery circuit was closed 

 through the tungsten lamp. This illuminated the selenium cell. The 

 method of procedure in the experiment was first to balance the selenium 

 cell in the bridge circuit, after it had recovered, and then set up keys 

 k2 and k4. The key k was closed by hand. Wlien the pendulum was 

 afterward released it closed key kl and illuminated the cell. When the 



