770 TEXT-BOOK OF PHYSIOLOGY 



acting interrupter placed in the circuit. With each make and break of the cir- 

 cuit the armature and style move alternately downward and upward. The 

 excursion of the style can be readily recorded on a traveling surface. The 

 character and number of the interruptions per second will determine 

 the character of the tracing. If they occur in a rhythmic manner, the tracing 



will be sinusoidal or wave-like 

 in form. If the time of inter- 

 ruption is of short duration as 

 compared with the time of 

 closure of the circuit, the trac- 

 ing will be a horizontal line 

 FIG. 345. SIGNAL MAGNET. with short vertical elevations at 



regular intervals. 



The Automatic Interrupter. The circuit may be interrupted by 

 vibrating reeds, tuning-forks, metronomes, etc. A well-known form of 

 vibrating reed is shown in Fig. 345. This consists of a metallic frame 

 carrying a coil of wire in the center of which there is a core of soft iron. To 

 the vertical part of the frame there is fastened the reed, the distal end of 

 which is bent to dip into an adjustable mercury cup. When in circuit the 

 current enters the coil, then flows into and through the frame and the 

 reed to the mercury, and thence back to the cell. On the closure of the cir- 

 cuit and the magnetization of the iron core the reed is withdrawn from the 

 mercury, the circuit broken, and the core demagnetized. The elasticity of 

 the spring returns it to the mer- 

 cury, when the circuit is again re- 

 stored. The reed may be so con- 

 structed that it will be raised and 

 lowered 50, 100, or 200 times a 

 second. The armature of the 

 signal magnet undergoes a corre- 

 sponding number of elevations 

 and depressions. If the reed vi- 

 brates 100 times in a second, the 



distance from crest to crest of the 



wave tracing will represent yfo of Fjc 346 ._ PAGE > S VIBRATING REED. (Reichert's 

 a second. Interrupters of various modification.) 



kinds have been devised which make and break the circuit from i to 250 

 times a second. 



Moist Chamber. In many experiments, it is necessary to keep the nerve 

 or muscle preparation in a uniformly moist atmosphere. To secure this, a 

 moist chamber is employed (Fig. 347). This consists of a hard-rubber 

 platform, supported by a piece of brass, which slides up and down a vertical 

 rod, and which can be clamped at any height. By means of a short lever 

 the vertical rod can be turned, carrying the platform from side to side. 

 The rod is secured to a firm iron base. 



Six double binding posts for the attachment of wires pass through the 

 platform . Near the side of the upper surface of the platform there rises a 

 vertical rod, carrying a clamp for holding the femur of a nerve-muscle prepa- 

 ration, as well as a horizontal rod for supporting at least three pairs of 

 non-polarizable electrodes. A groove around the outer edge of the platform 



