GRAPHIC REGISTRATION OF MUSCULAR CONTRACTION 



61 



In many cases it matters little which way this friction key is ad- 

 justed. Under certain conditions, however, it is desirable to stimulate 

 while the current is already under way and in closest proximity to the 

 muscle (B) rather than that it must first expend a certain amount 

 of its initial energy in passing all the way from the battery to the 

 preparation (A). Furthermore, if adjustment B is employe'd, the 

 muscle does not remain in direct connection with the battery, while 

 in A it remains in contact with the positive pole as long as the key 

 is kept open. This arrangement may at times give rise to unipolar 

 stimulation. Many other forms of keys have been devised. A 



FIG. 31. POHL COMMUTATOR. 



By moving the bridge B in the manner here indicated the current may be reversed 

 at the preparation M. The cross-bar of the bridge is insulated. 



very convenient one has been described by Morse. The current is 

 made by pressing upon a lever which is again forced upward by a 

 spring as soon as it is no longer pressed upon. In the mercury key, 

 contact is made by dipping the pointed end of the bridge into a 

 small porcelain cup filled with mercury. 



Commutators or pole-changers, such as have been devised by Pohl, are some- 

 times inserted in the circuit in order to be able to divert the current alternately 

 into two sets of electrodes and also to reverse its direction. A very useful type of 

 pole changer consists of a round block of wood containing six depressions filled 

 with mercury. The wires from the battery are connected with the two central 

 cups situated upon the opposite sides of the block. These cups contain the sup- 

 ports of a double rocking bridge which may be adjusted in such a way that the 

 current is diverted into the wires leading off from either pair of outside cups, or is 

 reversed by directing it across the central connections (Fig. 31). 



