SCIENCE AND PRACTICE. 



295 



10,000 units ; two branch or proportion resistance coils, of 

 each 10, 100, and 1000 units ; a coil of 10,000 units ; a galva- 

 nometer, two four-sided commutators, a battery commutator, a 

 contact key, a constant element (Daniell's), a battery of n 

 elements, and the necessary terminal screws for connecting 

 earth, galvanometer, and cables. 



The testing-board, fitted up with the more portable of 

 these articles, is shown in plan, in Fig. 139, with the 

 connections between the various members in dotted lines. 



Fig. 139. 



These connections are made of thick copper wire, well in- 

 sulated with gutta-percha; they are led from one point to 

 another, underneath, and their directions indicated by 

 strips of ebony or other wood, let into the top of the board, 

 and polished with it. In the laboratory, however, the 

 operator does better to make these connections in the air, 

 because, in the event of a leakage anywhere, he has them 

 more immediately under his inspection, and can eradicate 

 the disturbing cause with greater ease than if the wires were 

 fixtures. 



Fig. 140 shows a general plan of the same, arranged in a 

 more theoretical way. The resistance r (= 10,000 units) 



