38 



STIMULATORS 



The classical device for stimulating biological tissue is, of course, the induc- 

 tion coil. By interrupting the direct current flowing in a primary winding 

 an e.m.f. is induced into a mutually coupled secondary which drives stimulat- 

 ing current through the preparation* {Figure 38.1). The stimulating current 



* Output to 

 — '^ tissue 



Figure 38.1 



waveform has the form shown; a sharp rising phase followed by an expo- 

 nential decay. The magnitude of the shock depends on the mutual induc- 

 tance between the coils — one is arranged to slide in and out of the other — 

 and the time constant of the decay depends on the self inductance of the 

 secondary winding and the resistance of the preparation, being equal to the 



1 Low resistance 

 buzzer 



41/2V 



Dry 

 battery 



To external contacts 

 for single shock i 



Output 



Bell 

 transformer 



Shock 

 amplitude 



Figure 38.2 Tfie bell transformer is used with tlie windings reversed; that is, 

 the winding intended for connection to the mains is used as the secondary 



former divided by the latter. Two modes of operation are possible ; repetitive 

 stimulation, in which the contacts open and close automatically in the same 

 way as in an electric bell, and 'single shock', in which external contacts are 

 opened once only, at a definite instant, usually by the rotation of a kymograph. 

 Induction coil stimulators are expensive to buy and tedious to make. 

 Fortunately it is easy and cheap to make up a small unit which fulfils a 

 similar purpose from readily obtainable components as shown in Figure 38.2. 

 The operation is self-explanatory. At Cambridge these small stimulators are 



* A 'make' shock is also delivered when the primary circuit is closed; but since the 

 amplitude of the shock depends upon the rate of change of primary current, and since the 

 rate of change of primary current upon closing the circuit is limited by the self inductance 

 of the primary winding, the 'make' shock is much smaller. 



602 



