140 



PHYSIOLOGICAL PHYSICS. [Chap. XIIL 



Now r is the resistance to be measured, Re is the 

 resistance in the resistance box, which is read off at 

 once according to the number of plugs out (see page 

 136), and AS and SB are the resistances of the lengths 

 of the compensator wire on each side of the slider, 

 which are also known. Thus r, the resistance to be 

 measured, is given in terms of ne AS and SB, resis- 

 tances that are known. The resistance box is used, 

 because by pulling out or inserting plugs its resis- 

 tance can be varied at pleasure. According to the 

 resistance thrown in by it will be the position of the 

 slider, and thus, by varying it, it becomes easier to 

 adjust the proper position of the slider. 



Here again is the long compensator found a little 

 awkward on account of its length ; so a modification 

 of the round compensator (page 125) of Du Bois- 

 Reymond has been made by Prof. A. Christiani, of the 

 Berlin Physiological Institute. The modification con- 

 sists only in the addition of another connection. 



By comparing Fig. 72 with Fig. 65 (page 126), it 

 will be observed that the addition con- 

 sists in prolonging the compensator 

 wire beyond the point to which the 

 wire I is connected to it, and bring- 

 ing a wire from o to an additional 

 binding screw o' placed on the stand 

 of the instrument, by the side of the 



f 



screws Nps. I, u, in, and iv. (See 

 Fig. 65.) In fact, the two ends of 

 the platinum wi^e have each a double 

 connection. The screws o and n thus 



of DU afford means by which the unknown re- 

 pensator. sistance and the resistance box are put 

 into circuit, when the modified round 

 compensator is used for Wheatstone's bridge. The 

 mode of arranging it for this purpose is shown in 

 Fig. 73. The element E is connected by binding 



