220 ELECTRO-PHYSIOLOGY CHAP. 



second the foot of the first, that of the third the foot of the 

 second, and so on. Gradual approximation of the coils of the 

 induction apparatus caused each leg to twitch in succession. This 

 shows that " unipolar action in the vicinity of the metal pole is a 

 fallacy likely to occur even with scrupulous insulation." 



Under certain conditions the effect of "induction" is very bene- 

 ficial to unipolar excitation. Du Bois-Eeymond had occasionally 

 observed, when the finger was brought closer to a nerve-muscle 

 preparation attached only to one pole, that twitches appeared 

 which were not otherwise present at the same strength of current. 

 This as found by F. W. Zahn (60) is the case not merely 

 when the free end of the circuit is led off by contact with the 

 other hand, but even without this. Zahn modified this experiment 

 in many ways. He placed the preparation upon a round glass 

 plate, the under surface of which was covered with tinfoil to 

 within 10 cm. from the edge. On connecting one pole with the 

 leg, the other with the sheet of tin, tetanus appeared even with 

 weak currents. The same thing occurs with rather stronger 

 currents when the limb is disconnected from the secondary coil, 

 at the moment of leading off by contact, or when the free metal 

 pole is grasped in one hand while the other is brought near the 

 preparation. The experiment is still more successful if the 

 glass plate is evenly covered on both sides with metal, turning it 

 into a Franklin's Board. If one sheet is then connected with 

 one pole, the other with the nerve of the preparation, while the 

 leg hangs over the non-metallic glass edge and makes contact 

 with the other free pole, so that the circuit is interrupted only 

 by the glass disc between the two sheets of tinfoil, twitching 

 and tetanus are set up with even weak currents. The result is 

 the same on connecting one end of the induction circuit with 

 the lower sheet, while the other terminates in a plate of tinfoil 

 brought close to the leg. With the coils pushed home, there is 

 also stimulation when one end of the circuit is left free and 

 isolated, on bringing a plate of tinfoil sufficiently near to the 

 limb. 



Tiegel (60) connected one pole of an induction apparatus with 

 a gas-pipe, while the other terminated in an isolated metal plate 

 which could be moved towards a corresponding plate standing 

 opposite to it. The latter was in circuit with a glass plate 

 covered with tinfoil, on which the preparation was lying. Each 



