8 PEACTICAL PHYSIOLOGY 



amount of current which will pass through the nerve will be directly 

 proportional to the difference in potential between A and S, i.e. if the 

 fall in potential in the monochord is uniform, proportional to the 

 distance between A and S, being greater as S is moved away from A ; 

 it is also inversely proportional to the resistance of the circuit through 

 the nerve. But the resistance of this circuit may be considered con- 

 stant for all positions of S, since the resistance in the nerve itself is 

 enormously greater than that caused by any change in the length of 

 the monochord wire in the circuit. 



Although the Daniell cell is the most convenient source of current, 

 and its strength can be regulated by a rheochord, and although the 



FIG. 15. The induction-coil. 



make and break of a constant current do act as a stimulus to muscle 

 and nerve, it is often more convenient to use induced currents. These 

 are obtained by connecting a Daniell cell to an induction coil, and their 

 advantages are: (1) That being of extremely brief duration as com- 

 pared with the make of a constant current, they set up practically no 

 polarisation in the tissues (see page 78). (2) Having a comparatively 

 large E.M.F. and rapid development, as compared with the galvanic 

 current, they constitute a much more effective stimulus. For, the law 

 of excitation states that the effectiveness of a current as a stimulus 

 depends not only upon the total variation in its intensity, but also 

 upon the amount of such variation in the unit of time, i.e. the greater 

 the rapidity of the total variation, the more effective is the current 

 as a stimulus. 



The Induction-coil (Fig. 15) consists of two coils, of which the 

 primary is made up of a few turns of insulated thick copper wire with 

 only a small resistance. This is wound round a core of iron wire 

 to increase the number of lines of magnetic induction which pass 

 through it. The ends of the wire forming the primary coil are con- 

 nected with the top binding-screws 1 and 2 (Fig. 16). 



The secondary coil is made up of a large number of turns of insulated 



