2O EXPERIMENTAL PHYSIOLOGY 



coil is placed obliquely to the primary, and are strongest when the two 

 coils are again co-axial. This principle is embodied in Bowditch's 

 induction coil. 



8. Connect up the cell with the terminals t 3 , t* of the induction 

 coil (as in Fig. 23), introducing a simple key into the circuit. Set the 

 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, thereby making and breaking the 

 primary circuit (the mercury key being closed). 



9. 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 connexion with the secondary coil and the secondary circuit is 

 broken (unipolar induction). 1 It is on account of this possibility of 

 stimulating through only one pole that a simple key is never used in 

 the secondary circuit, but always a short-circuiting key, which is intro- 

 duced in the manner shown in Fig. 22. 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 cell circuit a simple key is always 

 used ; were a short circuiting key placed here the cell would rapidly 

 run down. 



10. Connect up a cell with the induction coil, using Helmholtz's 

 modification (Fig. 24). As in experiment 8, find the distance of the 

 secondary from the primary coil at which the induced shocks can just 

 be felt on the tongue, and determine that the make and break shocks 

 are now nearly equal by raising and lowering the spring by the hand. 

 Both are markedly diminished. 



1 The explanation of this is that the body acts as a condenser which becomes 

 charged and discharged through the electrode applied to the tongue. 



