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TEXT-BOOK OF PHYSIOLOGY. 



mercury, and thence back to the cell. On the closure of the circuit 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 magnetization of the iron core the reed is withdrawn from the mercury, the 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 mercury, when the circuit is again restored. The reed 

 may be so constructed 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 vibrates 

 100 times in a second, the distance from crest to crest of the wave tracing 

 will represent y^ of a second. Interrupters of various 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 



FIG. 349. MOIST CHAMBER. 



this, a moist chamber is employed. This consists of a hard-rubber plat- 

 form, 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 gan 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 

 preparation, as well as a horizontal rod for supporting three pairs of non- 

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

 receives a glass shade, which covers the whole. The air of the chamber 

 is kept moist by placing in it pieces of blotting-paper saturated with water. 



