182 



PHYSIOLOGICAL PHYSICS, [Chap. xvn. 



at either of two different points. If the muscle is 

 stimulated when the glass plate passes in front of the 

 lever, a curve of the muscular contraction is obtained. 

 Below the glass plate is placed a very ingenious 

 arrangement for insuring stimulation of the muscle at 

 a given time. It consists of two binding screws con- 

 nected by a movable arm c. Suppose the two wires 

 from a battery fixed to the screws; when the arm is 

 in contact the circuit will be closed, when the arm 

 is moved away the circuit will be broken. It is, in 

 fact, a key, and is placed in circuit with the battery 



and primary coil, while the 

 wires from the secondary 

 coil pass to the nerve. 

 Now a piece of brass pro- 

 jecting from the under 

 end of the pendulum is so 

 arranged as to come against 

 the movable arm. as the 

 pendulum swings, and sepa- 

 rate it from its contact 

 with one of the binding 

 screws, so that the current 

 is interrupted, and conse- 

 quently at that instant the 



Fig. 90. Arrangement for Pen- , . , . , , r 



duiuni Myograph. muscle is stimulated. Ihe 



detailed arrangements are 



shown in diagram in Fig 90. p is the smoked plate 

 of the pendulum, carrying the projection 3 for inter- 

 rupting the circuit. From the battery E a wire is 

 taken to binding screw 1, from which the current 

 passes across the lever 2, in connection with the other 

 binding screw, and so to the primary coil I of the 

 induction machine, and back to E. When the plate 

 swings across, 3 knocks 2 away from contact with 1, 

 making it take up the position indicated by the dotted 

 line, so that the circuit is interrupted. The wires 



