960 



Transactions of the American Institute. 



Effect of Temperature. 

 42. The resistance of copper increases about .208 per cent for each 

 deg. Faht.,* which is to be added or subtracted as the temperature 

 is below or above 60°. 



Example. 



If a wire has 22.73 ohms resistance at 70°, what will it have at 60° % 



22.73— (22.73 x .00208 X10°)=22.257. 

 The following table will be found convenient in making correc- 

 tions for temperature : 



Table for calculating the resistance of Copper at different tempera- 

 tures. 



Resistance of Batteries. 



43. In determining the interior resistance of batteries, the resist- 

 ance of the galvanometer coil used, if it has any appreciable resistance, 

 with the connections, must be known. In taking deflections where 

 accuracy is required, the direction of the current through the galvano- 

 meter should be reversed, and the mean of the two deflections taken ; 

 for we can scarcely fail to observe some difference. 



If we have no adjustable rheostat, we must be provided with one 

 or more standard coils, whose resistances are correctly known ; one 



* On Electric Measurement. By Latimer Clark, p. 68. 



