6;o 



COMPARATIVE ELECTRO-PHYSIOLOGY 



current. This latter is adjusted by means of a carbon 

 rheostat. In order to adjust the frequency of stimulation, 

 a toothed-wheel is fixed on clock-work, the normal period of 

 rotation of the axle being once in four seconds. This 

 speed, however, may be continuously adjusted within a 

 certain range by means of a regulating governor. When 

 the toothed wheel has sixteen teeth, the frequency of stimu- 

 lation is four times in a second. By inserting wheels with 

 different numbers of teeth, and by means of the regulating 



FIG. 405. The Sensimeter 



governor, it is possible to obtain any desired graduation of 

 frequency. 



The same experimental arrangement with slight modi- 

 fications, may be employed to give a series of thermal shocks, 

 For this purpose the electro-magnetic tapping arrangement 

 is removed from the apparatus, and the electro-thermic 

 stimulator attached to the circuit, instead of the tapper. The 

 frequency of the thermal shocks may be regulated in the 

 usual manner, their intensity being dependent on that of 

 the heating current, which is adjusted by the rheostat, 



