2IO 



PRACTICAL PHYSIOLOGY. 



[XXXVIL 



write exactly under the muscle-lever. Arrange, with its point exactly under 

 the other two, a Despretz chronograph or signal, in circuit with a tuning-fork 

 of known rate of vibration, and driven by means of a Grove's cell (fig. 129). 

 The three recording levers are all fixed on the same stand, which should 



EM 



Pt 



FIG. 129. Signal and Vibrating Tuning-Fork in an Electric Circuit. D. Drum ; 

 C. Signal ; EM. Electric tuning-fork ; Pt. Platinum contact. 



preferably be a tangent one, i.e., the rod bearing the recording styles can by 

 means of a handle be made to rotate so as to bring the writing-styles in con- 

 tact with the recording surface. 



On opening the secondary circuit and breaking the primary one, the muscle 

 contracts, and at the same time the style of the electro-magnet is attracted 

 and records the exact moment of stimulation (fig. 116). 



5. Despretz Signal (figs. 129, 130). This small electro-magnet has so little 

 inertia that, if it be introduced into an electric circuit, its armature, which 



FIG. 130. dspretz Electric Signal or Chronograph, as made by the Cambridge 

 Scientific Instrument Company. 



is provided with a very light writing point, vibrates simultaneously with 

 the vibrations of an electric tuning-fork introduced into the same circuit. 

 Arrange the signal and tuning-fork as in fig. 129. The drum must move 

 more rapidly, the more rapid the vibrations of the tuning-fork used. Use a 



