STIMULI AND THEIR ACTIONS 



409 



Du Bois-Reymond eaiployed the principle of the accessory circuit 

 ia his rheochord, an apparatus that serves to increase the intensity 

 in the circuit of a preparation as desired, by shunting definitely 

 measured resistances into an accessory circuit. For resistances 

 fine wires of definite length are employed, which can be intro- 

 duced one by one into the accessory circuit. The apparatus 

 (Fig. 193) consists in its essential parts of a thick bar of brass, the 

 continuity of which is broken at definite distances so that it is 

 really a series of separate metal blocks, which, however, can be 

 joined into a single bar by the introduction of metallic connecting 

 pieces. Each of these brass blocks 

 is joined to the adjacent block by a 

 very thin conducting wire of a 

 definite length, and upon the wire 

 that joins the first two blocks a 

 metallic slide can be shoved to and 

 fro, so that the wire can be shortened 

 or wholly cut out by shoving up the 

 slide. This whole apparatus is in- 

 serted as an accessory circuit into 

 the circuit of the preparation in such 

 a way that the two wire poles lead 

 from the source of the current to the 

 brass bar, and from there two other 

 wires lead to the preparation. If, 

 now, all the connecting plugs of the 

 metal blocks are inserted between 

 the blocks, so that the brass bar is 

 continuous, the condition that is re- 

 presented in Fig. 192, II is obtained. 

 A strong current passes through 

 the short circuit, because there is 

 little resistance there, while through 

 the long circuit a very feeble current 

 flows, because there the preparation 

 affords considerable resistance. But 

 the weak current going through the 



circuit of the preparation can be strengthened very conveniently by 

 increasing the resistances inthe short circuit, and this is accomplished 

 by shoving down the slide, farther and farther, so that the current 

 must pass through a constantly greater stretch of the first wire of 

 the rheochord, the distance being measured upon a scale. The 

 resistances can be strengthened still more by removing one by one 

 the connecting plugs between the metal blocks. The result is 

 that finally the current in the short circuit must traverse all the 

 wires of the rheochord, which with their fineness and length form 

 a very considerable resistance. But the more the resistances 



Fig. 193. — Ou Bois-Reymond's rheochord 



