582 PHYSIOLOGY. 



the conductor offering the smaller resistance, the brass key. If the 

 key is open, then the whole current passes to the nerve. This 

 method of using the key is known as "short-circuiting/" In using 

 the key to apply an induction or Faradic current to excite a nerve or 

 muscle, always use this method; that is, place a short-circuiting 

 key in the secondary circuit to prevent unipolar action. 



Mercury Key. Where a fluid contact is required the wires dip 

 into the mercury. It is used in the same way as the DuBois key, 

 for make-and-break shocks. 



Commutators. Pohl's commutator is used for sending (1) a cur- 

 rent into two different pairs of wires; (2) for reversing the direc- 

 tion of the current in a pair of wires; (3) it can also be used as a 

 mercury key. It consist of a round block of wood with six cups, 

 each containing a binding-screw. Between two of these stretches is a 

 bridge insulated in the middle. The battery is attached to the lead- 

 ing-in wires, and, as the bridge is rocked from one side to the other, 

 the current is sent through one or the other pair of wires. To 

 reverse the direction of a current, only one pair of leading-out wires, 

 besides the cell wires, is attached to the binding-screws of the mer- 

 cury cups. Then the cross bars are inserted, which change, the direc- 

 tion of the current on rocking the bridge. 



Induction or Faradic Currents. 



DuBois-Reymond's Induction Apparatus. It consists of a pri- 

 mary spiral of about 130 coils of a moderately thick silk-covered 

 copper wire, and of a secondary spiral of some 6000 coils of silk- 

 covered copper wire of a thickness of about a tenth of a millimeter. 

 The core inside the primary spiral is formed by a bundle of thin 

 iron wires, each carefully coated with shellac varnish. To graduate 

 the strength of the induced current of the secondary spiral, the sec- 

 ondary spiral is moved in a groove of wood from or towards the pri- 

 mary spiral, and the distance between the spirals is graduated in 

 centimeters and millimeters, or the secondary spiral is rotated as by 

 Bowditch. To make or close, or to break or open the circuit 

 coming from the cell through the primary spiral, the electro- 

 magnetic hammer of Neef is used to give us repeated shocks, 

 or the interrupted current. When single induction shocks are 

 used, the wires from the battery are connected with a key 

 and this, again, with the two terminals of the primary spiral. The 

 action of the coil of wires depends upon the fact that the strength 

 of a current running along a wire will be altered and an induced 



