IOWA ACADEMY OF SCIENCE 109 



Then keeping the time constant that the galvanometer was in the 

 circuit and substituting a known variable resistance in place of the 

 unknown data was obtained from which was plotted the calibration 

 curve shown in figure III. It will be observed from the calibration 

 curve that for changes of resistance ranging in value from 500,000 to 

 564,000 ohms the deflection is almost a linear function of the change 

 in resistance. For greater changes, however, the deflection changes very 

 rapidly and becomes inflnite when the change of resistance equals the 

 original resistance in that arm of the bridge. It is interesting to com- 

 pare the curve with the curve in figure II. Both are deflection-resist- 

 ance curves, but in iigure II the change in resistance begins with zero 

 while in figure III they begin with about 500,000 ohms. The resistances 

 are of entirely different order of magnitude in the two instances. Using 

 the values of the deflections obtained with the selenium cell shown in 

 the accompanying table, we are able to determine the change of re- 

 sistance from the calibration curve. 



Other applications of this method may be found where it is neces- 

 sary to vary the interval during which the galvanometer is the circuit 

 and also the E. M. F. of the battery. This method would probably adapt 

 itself to determining the change of resistance with application of heat 

 in a bismuth spiral, change of resistance of certain metals, as manganin, 

 and silver-sulphide, under pressure. The method is probably as ac- 

 curate and as easy to manipulate as any method that has been devised 

 for measuring rapidly fluctuating resistances. 



