STIMULATION OF MUSCLE AND NERVE 105 



Fig. 27. The signal magnet ; the actual size. 



The Electro-magnetic signal. 1 A protecting raetal 

 box (Fig. 27), open at the front and ends, contains a 

 strong magnet, the armature of which is mounted upon 

 a steel spring. An accurate fine adjustment screw reg- 

 ulates the excursion of the armature. One binding 

 post is mounted upon the metal box, the other is insu- 

 lated by a rubber block. This signal, in circuit with a 

 vibrating tuning fork, will record one hundred double 

 vibrations per second. In the primary circuit of the 

 inductorium it will record the make and break of the 

 current without after-vibrations which are prevented by 

 lead foil placed on the spring where it strikes the limit- 

 ing screw. Residual magnetism is obviated by parch- 

 ment paper, fastened to the spring with shellac at the 

 point where the spring would touch the core of the 

 magnet. The handle is long enough to bring the writ- 

 ing point directly above or below the writing point of 

 the muscle lever clamped to the same iron stand. 



The metal box is of soft iron and serves as an 

 extension of the magnet core, thus completing the 

 magnetic circuit, and doing away with a second spool. 

 The magnetic power is further improved by boring 

 out the core, which is then " softened " by heat. 



1 First Catalogue of Harvard Apparatus, September, 1901, p. 46. 



