PHYSIOLOGIC APPARATUS. 



685 



extent but also the duration, the form, and the rate of recurrence of any 

 given movement. 



The Extent of a Movement. As the lever not only takes up and repro- 

 duces a movement, but at the same time magnifies it, it is essential that 

 the degree of magnification be known, in order to determine the actual 

 extent of the movement. The magnification of the lever is readily deter- 

 mined by dividing the distance between the axis of the lever and its writing 

 point by the distance between the axis and the point of attachment of the 

 structure, and then dividing the height of the tracing by this quotient. 

 The final quotient 

 represents the extent 

 of the movement. 



The Time Rela- 

 tions oj a Movement. 

 When recorded in 

 the form of a curve, 

 the duration of the FlG - 347 SIGNAL MAGNET. 



entire movement, or 



of any one portion of it, can be determined by means of a time marking 

 or chronographic apparatus, consisting of (i) a small signal magnet 

 provided with a movable armature, to- which is attached a writing style; 

 (2) an automatic interrupter; and (3) an electric cell. 



The Signal Magnet. The magnet (Fig. 347) is actuated by the electric 

 current made and broken at regular and known intervals by an auto- 

 matically-acting interrupter 

 placed in the circuit. With 

 each make and break of the 

 circuit the armature and style 

 move alternately downward 

 and upward. The excursion 

 of the style can be readily 

 recorded on a traveling sur- 

 face. The character and num- 

 ber of the interruptions per 

 second will determine the 

 character of the tracing. If 

 they occur in a rhythmic 

 manner, the tracing will be 

 sinusoidal or wave-like in 

 form. If the time of interruption is of short duration as compared with 

 the time of closure of the circuit, the tracing will be a horizontal line 

 with short vertical elevations at regular intervals. 



The Automatic Interrupter. The circuit may be interrupted by 

 virbating reeds, tuning-forks, metronomes, etc. A well-known form of vibrat- 

 ing reed is shown in Fig. 348. This consists of a metallic frame carrying a 

 coil of wire in the center of which there is a core of soft iron. To the 

 vertical part of the frame there is fastened the reed, the distal end of which 

 is bent to dip into an adjustable mercury cup. When in circuit the current 

 enters the coil, then flows into and through the frame and the reed to the 



FIG. 348. PAGE'S VIBRATING REED. (Reich- 

 ert r s modification.) 



