PRACTICAL PHYSIOLOGY 19 



Neef 's hammer in vibration. The electrodes from 

 the secondary coil are to be applied to the tongue, 

 and the distance of the secondary from the primary 

 coil found at which the induced shocks can just be 

 felt. Determine that these are the break shocks 

 by raising and lowering the hammer by the hand, 

 and thus slowly making and breaking the primary 

 circuit (the key, K*, being closed). 



9. Unipolar induction. Detach one of the 

 wires of the electrodes from the secondary coil so 

 that only one electrode is connected with that coil. 

 Slide the coil home. Pass a strong current through 

 the primary coil and set Neef 's hammer going as 

 in the last experiment. It will be found that shocks 

 are faintly felt by the tongue, although only the 

 one electrode is in connection with the secondary coil 

 and the secondary circuit is broken. It is on account 

 of this possibility of stimulating through only one 

 pole that a simple key is never used in the second- 

 ary circuit, but always a short-circuiting key, which 

 is introduced in the manner shown in Fig. 18. No 

 shocks can pass to the electrodes when the key is 

 closed, since the coil is then short-circuited ; only 

 when the key is open are the shocks conducted to 

 the electrodes. On the other hand, in the primary 

 or battery circuit a simple key must always be 

 used ; were a short-circuiting key placed here the 

 battery would rapidly run down. 



10. Connect up a battery with the induction coil, 

 using Helmholtz's modification (Fig. 19). As in 

 experiment 8, find the distance of the secondary 

 from the primary coil at which the induced shocks 



