VARIOUS TYPES OF RESPONSE 67 



period of such contact would be impracticably long — 

 more than 15 seconds. 



So continuous a tetanisation would undoubtedly fatigue 

 or even injure the tissue. The contact made by a seconds- 

 hand, again, though sufficiently brief, would have the 

 serious defect that its movements were jerky and would 

 therefore make the duration of contact unequal. 



I succeeded in overcoming these difficulties by using a 

 released revolving disc, which could be made to complete 

 an electrical circuit for any definite short period that was 

 required. For this I employed a phonograph motor, an axis 

 of which, carrying a disc, could be adjusted to revolve once 

 in a second. 



This disc is usually held stationary by a lever-clutch, 

 and can be started only by the pressure on it of the re- 

 volving rod of the Periodic Starter. It is re-arrested after 

 one complete revolution and is not again released till the 

 next rod comes into position, after an interval of 10, 15, or 

 20 minutes, as the case may be. There is also attached 

 to the disc a sector whose arc is one-tenth the circum- 

 ference of the circle. This sector, during the revolution, will 

 press against a closing-key, the period of closure being then 

 •1 second. By increasing or diminishing this arc the time of 

 closure, and with it the duration of the tetanising shock, can 

 be correspondingly changed. It is necessary that the sector 

 should, at the moment of the release, be at the greatest 

 possible distance from the closing-key. By the time it 

 reaches this key it will have acquired a constant and definite 

 velocity. Thus the periods of closure, and consequent 

 duration of the exciting-shock, will be identical in successive 

 experiments. 



There is another possible source of variation which 

 must be guarded against. The electrodes of the secondary 

 coil are connected with the plant by means of moistened 

 threads. These threads, in long-continued experiments, 

 may become more or less dried up, the electrical resistance 

 being thus increased in an unknown manner. The intensity 



F2 



