174 PHYSIOLOGICAL PHYSICS, [Chap. xvi. 



them, and from them to the other binding screw. The 

 electro-magnet attracts the limbs of the fork, and 

 so contact is broken between the platinum wire and 

 the platinum plate, and the current is interrupted. 

 The fork is restored to its original position by its 

 elasticity, and re-establishes the contact, so that the 

 bobbins again become magnetic, attract the fork, 

 and again contact is broken. Thus the fork is made 

 to vibrate, the current is alternately interrupted and 

 established, and the number of these interruptions 

 corresponds to the number of vibrations of the fork. 

 To connect this apparatus with the stylet, a 

 wire from the battery is led to one of the binding 

 screws of the fork ; from the other screw a wire 

 goes to one screw of the stylet, and from its re- 

 maining binding screw a wire goes back to the 

 battery. Both are now in circuit. The current 

 traversing the stylet is interrupted a certain number 

 of times by the fork, and its fine quill point can thus 

 be caused to vibrate in unison with the fork. When 

 this stylet is fixed to a support and brought against 

 the smoked surface of a revolving cylinder, intervals of 

 time are marked according to the number of vibrations 

 per second of the tuning fork. The fork can usually 

 be removed from its support, and another one vibrating 

 a fewer or greater number of times can be substituted, 

 so that different rates of movement can be communi- 

 cated to the vibrating stylet. Thus the -g^th of a 

 second can be measured with ease, and the measure- 

 ment of even more than half this interval has been 

 accomplished. Such an instrument as has been 

 described is called a CHRONOGRAPH, or time writer. 



