172 PHYSIOLOGICAL PHYSICS. [Chap. xvi. 



magnets, and leaves by the other binding screw. The 

 keeper K has attached to it a lever b, which is drawn 

 down by the magnets. When the current is in- 

 terrupted the keeper is withdrawn by the elasticity of 



the spring c, and the lever flies 

 up. Now, if the current of this 

 electro-magnet be interrupted and 

 then established by the movement 

 of a seconds pendulum, the lever 

 brought against the surface of a 

 revolving cylinder will mark 

 seconds. 



The clock may be arranged to 



Fig. 84. Electric Sig- , i_ T_ ir j i j.i 



na i. interrupt each hair second, and the 



lever will then mark half seconds. 

 Further, the metronome figured on page 71 can be 

 adapted to the electro-magnet, and the number of its 

 movements per minute will be reproduced by the 

 electro-magnet. Still more minute intervals of time, 

 however, can be registered by the use of tuning forks. 

 If to one limb of a fork a fine stylet be attached, and 

 if the fork be caused to vibrate and the stylet be 

 brought against the surface of a revolving cylinder, the 

 moving limb will write a series of curves, each curve 

 corresponding to a to-and*fro movement of the limb of 

 the fork. Suppose the limb vibrates 100 times per 

 second, then each curve will equal y^^th of a second. 

 Another fork vibrating 200 times per second, will 

 write ^iro'kh of a second, and so on. Such rapid 

 vibrations cannot, however, be well recorded without 

 the use of the electro-magnet. The method of 

 adapting this to the tuning fork has been devised 

 by Marey. Fig. 85 shows one part of the apparatus. 

 It is an electro-magnet of two bobbins with soft iron 

 cores ; on each is a soft iron keeper bb ; between the 

 keepers a triangular interval is left, which is occupied 

 by a wedge-shaped piece of soft iron c, supported on 



