100 



and the other materials were bought of Eiuier iV: Amend as being cdiemically pure. 

 The cells are easily made, and can be set up by anyone without much trouble. 



The method adopted for comparing these cells one with another was a modi- 

 tication of the potentiometer method used by Professor Kahle, and was as follows: 



ri^.2. 



The current from a single storage cell A \ Fig. 2) passes through an ordinary 

 resistance box B^ and through a wire resistance B, made of German silver, with 

 sliding contact capable of continuous variation for fine adjustment. At the mer- 

 cury commutator M the circuit is divided. The first branch passes through £, 

 then through the wire c back through c/ and M to the storage cell A. The second 

 . branch of the circuit passes successively from M through the transfer switch S, 

 the variable resistance i2', the sensitive galvanometer G, back again to S, and 

 thence through W to M. The resistance box B was made especially for this pur- 

 pose and consists of seven coils of wire, having the resistances of 10, 15, 50, 100, 

 300 and 500 ohms approximately. These dip into a dish of kerosene, so that 

 their temperature can be measured more readily. C is a German silver wire 1,122 

 mms. long, stretched tightly over a boxwood meter bar. The resistances of both 



