ELEMENTAEY EXPERIMENTAL PHYSIOLOGY 23 



tracing will run over itself. The number of complete vibrations per 

 second and the time value of each will depend upon the note of the 

 fork. The most useful fork is one that gives 100 complete vibrations 

 per sec. When more rapid vibrations are required the above method 

 is not suitable, because the vibrations of a fork of a higher note cease 

 so soon after a single tap. 



In order to obtain a time-tracing in Tj^ths or less of a second, it is 

 necessary to use 



(2) A Chronograph or time-marker, which records on a drum the 

 number of times per second a current through it is made and broken by 



FIG. 28. A time-marker. 



another special piece of apparatus. The chronograph (Fig. 28) consists 

 essentially of an electro-magnet, which, when the current through it is 

 made, attracts and pulls down a metal lever carrying a writing point. 

 When the current through the electro -magnet is broken, a spring at 

 the other end of the lever raises the writing point. 



The apparatus used to make and break a current through the chrono- 

 graph at any definite known rate is a tuning-fork of the corresponding 

 note. To one prong of the fork is attached a platinum wire which, 



Fio. 29. A tuning-fork with electro-magnet. 



with each complete vibration of the fork, makes and breaks the chrono- 

 graph circuit by touching and receding from a brass contact or mercury 

 cup (Fig. 29). The tuning-fork, when once started vibrating by a tap, 

 is kept vibrating automatically by an electro-magnet in the same circuit 

 (Fig. 30). Thus, when the platinum wire touches the mercury cup the 

 battery current is made through the chronograph and the writing point 

 is pulled down; at the same time the current is made through the 

 other electro-magnet, which attracts the tuning-fork and pulls the 

 platinum point away from the mercury. Both electro-magnets now 

 cease to act, the writing point of the chronograph is pulled up by the 



