THE POTENTIOMETER. 



27 



Therefore by subtraction we have 0.000994 = 0.975 x - 



Hence the fall of potential from one end of the slider to the other is 

 0.00102 volt. 



The scale of the slider was calibrated with the help of a small potentio- 

 meter box, carefully standardized, containing ten 2-ohm coils. The extremi- 

 ties of the slider were soldered to the terminals of this box by heavy copper 

 wires ; after equilibrium of temperature was assured, the poles of a battery 

 were attached at these points. Then with the help of a sliding contact con- 

 nected through a galvanometer to the various pegs on the box, nine points on 

 the slide wire were found where no deflection was noted. These could be 

 referred to the known resistances of the bridge, and their error determined. 

 The determination was then repeated with the box reversed. Because the 

 slight corrections thus found were regular, it was assumed, as seems per- 

 missible, that the variation of the wire between these points proceeded 

 at a uniform rate. The two errors of the slider can now be combined and 

 corrected for together. 



These values were used in the work on zinc. Eight months elapsed be- 

 tween the work on zinc and that on cadmium, and after that time the slide 

 wire showed traces of corrosion. It was polished with very fine sandpaper 

 and then restandardized with the same precautions as before. 



