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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



curve written by the muscle. The rate of movement of the recording surface 

 can be registered by an instrument called a chronograph. 



The chronograph (g, Fig. 34), consists of one or two coils of wire wound 

 round cores of soft iron, and a little lever bearing a strip of iron, which is 

 attracted to the soft-iron cores whenever they are magnetized by an elec- 

 tric current flowing through the coils of wire about them. When the current 

 ceases to flow and the iron ceases to be magnetized, a spring draws the lever 

 away from the iron. Many of the instruments employed for this purpose are 



FIG. 34. Method of interrupting an electric circuit by a tuning-fork, and of recording the interrup- 

 tions by means of an electro-magnet : a, battery ; 6, tuning-fork, with platinum wire at the extremity 

 of one of its arms, which with each vibration of the fork makes and breaks contact with the mercury 

 in the cup below; c, mercury cup; e, electro-magnet which keeps the fork vibrating; g, chronograph. 

 The current from the battery a, passes to the fork &, then, by way of the platinum wire, to the mercury 

 in cup c, then to the binding-post d, where it divides, a part going through the coils of wire of the 

 chronograph g, and thence to the binding-post/, the rest through the coil of wire of electro-magnet 

 e, and then to the post /, from which the united threads of current flow back to the battery. The 

 electro-magnet e keeps the fork in vibration, because when the platinum wire enters the mercury 

 at c, the circuit is completed and the electro-magnet magnetizes its soft-iron core, which attracts the 

 arms of the fork, and thus draws the wire out of the mercury and so breaks the circuit. When the 

 current is broken the fork, being released, springs back, dips the wire into the mercury, and by 

 closing the circuit causes the process to be repeated. 



very delicate, and are capable of responding to very rapid interruptions of the 

 current. The electric current is made and broken at regular intervals by a clock, 

 tuning-fork (6, Fig. 34), or other interrupting mechanism, and the lever of the 

 chronograph, which has a writing-point at its free end, moves correspondingly 



FIG. 35. Myogram from gastrocnemius muscle of frog ; beneath, the time is recorded in 0.005 second : 

 a, moment of excitation ; b, beginning of contraction ; c, height of contraction ; d, end of contraction. 



and traces an interrupted line on the recording surface of the myograph (see 

 Fig. 35). The space between the succeeding jogs marked by the chronograph 

 lever is a measure of the amount of the surface which passed the point of the 

 chronograph in one second, ^ second, or ^ second, as the case may be. 



