654 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



junctions within the prism were soldered together, and to a copper wire 

 leading to the (copper) wire of a potentiometer. The copper ends of the 

 couples led to a mercury switch by which any one of them, or any pair 

 pitted against each other, might be quickly connected with tlie jjoten- 

 tiometer. On its way from this switch to the cold junctions in C through 

 the potentiometer wire the current encountered only copper. By means 



Figure 3. 



of a somewhat elaborate standard potentiometer, not shown in the dia- 

 gram, the resistance, R, in the potentiometer circuit could be so adjusted 

 that a millimeter on the potentiometer wire corresponded to any desired 

 small potential difference, such as one micro-volt or one-tenth of a micro- 

 volt. The potentiometer wire, which was 0.25 mm. in diameter, could 

 be changed in a few seconds for new wire if the old became dented or 

 stretched. An iron casting ^(Figures 2 and 3), accurately planed below 

 and turned true above, formed the bottom of the ice-box : it could be 



