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EXPERIMENTAL PHYSIOLOGY. 



from the battery are connected with a key, and this again with 

 the two terminals of the primary spiral, S" and S'". 



Fig. 46. Induction apparatus of du Bois-Reymond. R', Primary; R", 

 secondary spiral ; B, board on which R" moves ; I, scale ; + - , wires 

 from batteiy ; P', P", pillars ; H, Neef's hammer ; B', electro- 

 magnet; S', binding screw touching the steel spring (H); S" and S"', 

 binding screws to which are attached wires when Neef's hammer is 

 not required. 



3. Ordinary Hand Electrodes. Take two pieces of flexible gutta- 

 percha'coatedwire(No. 20) 60 cm. long, and two pieces of thick glass 

 tubing 8 cm. long, and with a bore sufficient to admit the wire. 

 Push a wire through each tube, and allow the end of the wire to 

 project 3 cm. beyond the tube, scrape the gutta-percha off the 

 free ends of both wires. Fix the wires in the glass tube with 

 sealing-wax, and with a well-waxed thread bind the two tubes 

 together. A very handy holder is made by thrusting two fine 

 insulated wires (No. 36) through the bone handle of a crotchet 

 needle. 



4. For some purposes " shielded electrodes " are used, i.e., the 

 platinum terminals are exposed only on one side, the other 

 being sunk in a piece of vulcanite (Fig. 61, E). 



5. Du Bois-Reymond's Electrodes (Fig. 47). The two wires end 

 in triangular pieces of platinum (P), which rest on a glass plate. 

 The whole is supported on a stand (V), and can be moved in any 

 direction by the universal joint (B). 



