NERVE STIMULATION 17 



ary coil of the inductorium ; close the key ; connect the 

 primary coil by an open in-circuit key with the cell (cp. 

 Fig. 4). With two pins (Fig. 1 6) fix the electrodes a 

 little above the level of the tissue, and rest the nerves 

 on them, covering the whole with a small piece of blot- 

 ting paper moistened with Ringer's fluid to prevent the 

 nerve from drying. Put the index of the secondary coil 

 at 15. Open the short-circuit key. 



Close the primary circuit ; the muscles contract. Open 

 and close the key in the secondary circuit ; this of course 

 has no effect, since there is no current in the secondary 

 coil as long as the primary current remains constant* 

 Open the key, and break the primary circuit; there is 

 again contraction. Determine the position of the second- 

 ary coil at which contraction is first obtained (i) on 

 making, (ii) on breaking the primary current. 



Close the short-circuit key and push the secondary 

 coil over the primary so that very strong induced cur- 

 rents will be obtained. Make and break the primary 

 current, no contraction is caused. The resistance of the 

 nerve is so great compared with that of the key that 

 practically no current passes through the nerve (cp. 

 Exp. d). 



b. Series of make or break induction currents 

 only. Put the secondary coil at 15. Close the short- 

 circuit key in the secondary circuit, make the current 

 in the primary circuit, open the key in the secondary 

 circuit, break the current in the primary circuit, and 

 repeat; thus contractions are obtained at the break only 

 of a primary current. 



In a similar manner obtain contractions at the make 

 only of the primary current. 



