8o PHYSIOLOGICAL PHYSICS. [chap. vin. 



resistance. Between the fifth and sixth plates is a 

 similar wire, but five times the length of the 

 first, while that between the sixth and seventh is ten 

 times that of the first. Suppose, therefore, a current 

 enters at #, if the slider is pushed close up to the 

 brass plates, and all the plugs are in, no resistance 

 will be offered to the passage of the current 

 straight across to the binding screw at/; but then, 

 by pushing the slider up towards the knife edge, and 

 afterwards by removing one plug after another so as 

 to cause the current to traverse the German silver 

 wires also, a gradually increasing amount of resistance 

 may be interposed in the pathway of the current. 

 The resistance may also be varied at pleasure by 

 altering the position of the slider, inserting some 

 plugs or removing others. 



The rlieocord must always foe connected 

 in sBiort circuit. Thus, in Fig. 42, let E be the 

 element ; bring two wires from it, one, n l to a, at one 

 side, the other, n 2 to/, at the other side, of the rheo- 

 cord, interposing a simple key x on the way. From a 

 take a wire h l to the apparatus, App, to which the 

 current is to be sent (the nerve to be electrotonised), 

 and from App bring a wire h 2 back to the rheocord 

 at / Now when the current from the battery reaches 

 a it has two pathways ; it may go straight through 

 the rheocord and back to the battery (be short- circuited, 

 in fact), or it may go off by h l round App, and back 

 by h~ to / thence by n 2 back to the battery, or part 

 may go through the rheocord, and part round App. 

 The course it takes depends on the resistance of the 

 two circuits. "When the slider is home, and the 

 plugs in, the resistance of the rheocord is practically 

 nil as compared with that offered by even a small 

 fragment of a nerve, and consequently all the current 

 will be short-circuited. 



By then moving up the slider, and; if necessary, 



