SCIENCE AND PRACTICE. 



307 



wound upon a wooden reel, c A ; their ends terminate in thick 

 copper connections, which dip into the mercury cups a a 1} c 

 and c v The standard resistance s is wound upon a similar 

 reel, and its ends connected to the amalgamated copper wires, 

 which terminate in mercury cups s s t . n is the resistance 

 whose length is to be adjusted. D is a commutator consisting 

 of two parallel arms of copper connecting the mercury cups d 

 with d lt and /with/u or d with/ and d t with/. E is a gra- 



,V>f -<^v| 



~o o-. 



Fig. U6. 



duated scale, underneath which the adjusting- wire w x is 

 stretched, and with which the sliding brass piece H is in 

 contact. The contact-key is of original construction ; its duties 

 are to close the battery circuit first and then the galvano- 

 meter circuit. It is made of three brass springs, 1, 2, and 

 3, Fig. 147, each insulated from the other at K, and connected 

 by screws with the opposite corners of the bridge ; 1 and 2 



x2 



