12 



EXPERIMENTAL PHYSIOLOGY 



If the crossed wires are removed the Pohl can be used as a 

 switch for diverting a current into one or other of two circuits 

 (fig. 16). 



Other commutators have friction-contacts in place of mercury : of these 

 the simplest are Waller's (fig. 17), which has a rotating action; and Malcolm's 

 (fig. 18), which has a sliding action. The principle is the same as the mercury 

 commutator, and it is often convenient to dispense with mercury. 



Rheochords. A rheochord is an apparatus for dividing a constant 

 current by offering a circuit of relatively small resistance which is 

 capable of being varied so that a variable part only of the current 

 shall pass through the experimental circuit. It usually consists of a 

 german-silver or platinum-iridium wire of a certain known resistance 



FIG. 19. DIAGRAM OP BHEOCHORD. 



(e.g., 20 ohms), to the ends (fig. 19, a and 6) of which the battery poles 

 are connected ; a certain difference of potential is thereby produced 

 at the two ends of the wire. With one of these ends (b) another wire 

 is connected ; this forms part of the experimental circuit through 

 which a portion of the battery current is to be conducted ; this circuit 

 is completed through a wire attached to a rider (r) which slides along 

 the rheochord wire. 



When r is in contact with b the whole difference of potential between a and 

 b which depends upon the E.M.F. of the battery and the resistance of the 

 rheochord wire relative to that of the experimental circuit- is operative in pro- 

 ducing a current through the preparation. When r is at the middle of the 

 rheochord wire only one-half of this difference of potential comes into play, 

 and so in proportion to the distance between a and r as compared with the 

 whole length of the wire. Thus if the wire be 100 centimeters long and r be 



