MUSCLE AND NERVE 



27 



tion of a small vulcanite cup which is partially filled with mercury. 

 To the sides of the wooden base are attached two brass rods. The 

 inner ends of these dip into the mercury, while their outer ends are 



FIG. 4. MORSE KEY. 



connected by means of binding-posts with the poles of the battery. 

 One of these rods is jointed so that its inner end may be dipped into 

 the mercury or removed from it at will, thereby making and breaking 

 the current. The friction key consists of 

 a vulcanite base to which are attached 

 two oblong bars, II and III, each of which 

 is equipped with two binding-posts. Bars II 

 and III are joined by a third bar, IV, which 

 is jointed at one end and may be elevated 

 at its other end, thereby breaking the 

 contact between II and III. On lowering 

 this bridge the current is made, while on 

 raising it it is broken. The automatic key 

 presents many forms. It consists, as a 

 rule, of a rotating disk to which are at- 

 tached a number of contacts. Since the 

 speed of the disk and the position of these 

 contacts may be varied, it is possible to 

 make and break the circuit without effort 

 and at definite intervals. 



3. Stimulating Electrodes. The elec- 

 tric current may be conveyed through 

 living matter by simply applying the ends 

 of the copper wires to its surface. The 

 closure of the key then completes the 

 circuit from carbon to zinc. Most com- 

 monly, however, the ends of the copper 

 wires are brought close together and soldered to platinum points. They 

 are then placed within a narrow piece of hard rubber, so that they can 

 be conveniently handled. This form of electrode cannot be used for 

 a long time, because the conduction of an electric current through moist 



FIG. 5. FRICTION KEY. 



