INDUCTION MACHINE ELECTRODES. 139 



Fig. 59). At other times it is desired to reverse the direction 

 of a current. This is done by Pohl's 

 commutator with cross-bars, or by means 

 of Thomson's reverser. 



13. Thomson's Reverser (Fig. 45) may 

 be used to reverse the direction of a 

 constant current. The wires from the 

 battery are connected to the two lower, 

 and those from the electrodes to the 

 upper binding screws. The binding 

 screws are four in number, and placed 

 behind the circular disc seen in the 



figure. When the handle is horizontal m, , g 



TiP T <y r ,1 Ihomson s Reverser. 



the current is shut on from the 



electrodes, while the direction of the current is reversed by 

 raising or lowering the handle. This instrument is used solely 

 for reversing the direction of a current. 



LESSON XXIV. 

 INDUCTION MACHINE ELECTRODES. 



1. Induced or Faradic Electricity is most frequently employed 

 for physiological purposes. 



2. Induction Apparatus of du Bois-Reymond. In Fig. 46 the 

 primary coil (E/) consists of about 150 coils of thick insulated 

 copper wire, the wire being thick to offer slight resistance to the 

 galvanic current. The secondary coil (B/') consists of 6000 turns 

 of thin insulated copper wire arranged on a wooden bobbin ; the 

 whole spiral can be moved along the board (B) to which a milli- 

 metre scale (I) is attached, so that the distance of the secondary 

 from the primary spiral may be ascertained. At the left end of 

 apparatus is Wagner's hammer as adapted by Neef, which is an 

 automatic arrangement for opening and breaking the primary 

 circuit. When Neef s hammer is used to obtain what is called 

 an interrupted current, the wires from the battery are connected 

 as in the figure, but when single shocks are required, the wires 



