688 Mr. S. 0. Laws on the Change of Resistance of Metals 



Keeping these resistances fixed the deflexion due to a 

 reversal of the current (about -J-q ampere) through the 

 bridge arrangement was observed. This observation was 

 made first with no field on, then with a current of 4*4 

 amperes through the coils of the magnet ; it was then 

 repeated with the field zero, and so on as shown in the above 

 table. 



The deflexions on reversal were obtained by observing the 

 position of the image of an incandescent- lamp fiiament on a 

 scale some 3 metres distant from the galvanometer, first with 

 the current through the bridge arrangement direct, then 

 reversed, and finally in its original direction, the deflexion 

 on reversal being obtained by subtracting the second reading 

 from the mean of the first and third. These successive 

 readings of the position of the image of the lamp filament 

 were taken at intervals of 15 seconds. The galvanometer 

 was a D'Arsonval instrument, whose resistance was 60 ohms 

 and sensitiveness 2*5 x 10 -9 ampere per scale-division. The 

 average of the above readings together with those made 

 under similar circumstances with other currents are tabulated 

 below. The readings were standardized by observing in tlie 

 same way the deflexions on reversal when the resistance X 

 was changed from 1520 to 1530 ohms, there beino- no current 

 in the magnet coils. 



The deflexions on reversal obtained in this experiment are 

 as follows : — 



X. 



1530 



Deflexion on 



Deflexion due to 



reversal. 



change of resistance. 



75 





1520 



-12-5 



91'7 



1530 



83-5 



95-5 



1520 



-11-5 



96 7 



1530 



87 



97-5 



1520 



- 95 



95 



1530 



84 



92-5 



1520 



- 7-5 



915 



1530 



84 



946= mean. 



With X = 1530 the equivalent resistance of X and Y was 



184-655. 

 With X=1520 the equivalent resistance of X and Y was 



184-509. 

 So that a change of -146 in the resistance of this arm pro- 

 duced a galvanometer deflexion of 95 divisions. 



