METHODS OF STIMULATION 31 



allowing successive stimuli to be maintained constant, or 

 to be increased in a known manner. 



The induction coil consists of a primary made of a 

 few turns of thick wire enclosing a bundle of soft iron 

 wire — thus forming an electro-magnet — and of a secondary 

 consisting of a larger number of turns of thin wire. The 

 secondary coil can be made to approach or recede from 

 the primary, by means of a slide. When a current is 

 suddenly started in the primary coil, by pressing a key an 

 instantaneous make-induction-current is induced in the 

 secondary. When, by releasing the key, this current 

 is broken, an instantaneous break-induction-current, whose 

 direction is opposite to that of make, is induced in the 

 secondary. Owing to the greater suddenness with which 

 the break is effected, the intensity of the break-shock is 

 greater and of shorter duration than that of the make-shock. 

 The intensity of either make- or break-shock may be in- 

 creased by bringing the secondary nearer the primary. 

 We can obtain successive uniform shocks, of either make 

 or break, by maintaining the distance between the two 

 coils constant. 



We may subject the tissue to shock of either make or 

 break at will by employing an additional short-circuiting 

 key. When this key is down, the shock from the secondary 

 coil is practically diverted across the better conducting- 

 path provided by the key, so that for practical purposes 

 none passes through the plant tissue. If it is desired to 

 cause successive excitations by make-shock only, then the 

 short-circuit key is raised when the primary circuit is made, 

 and pressed down when this is broken. For exciting by 

 break-shock, the short-circuit key is pressed when the primary 

 is made and raised when the primary is broken (fig. 11). 



Effects of Make- and Break-shock 

 In the response of animal tissue it is well known that 

 while single induction-shocks are effective in the case of 

 quickly reacting skeletal muscles, they induce hardly any 



