ELEMENTARY EXPERIMENTAL PHYSIOLOGY 7 



ling wax (Figs. 10 and 11). A form of electrode sometimes very useful 

 is made by soldering the free end of each wire to the head of a needle. 

 The Rheochord is used to alter the strength of a constant current to 

 be sent through a muscle or nerve. In its simplest form it consists of 



Fio. 12. Simple form of monochord. 



a single straight or zig-zagged wire with a binding- screw at either end 

 and a movable contact between them (Fig. 12). If a Daniell cell be 



A + 



FIG. 13. To illustrate the principle of the monochord. 



connected to the two ends of the monochord A and B (Fig. 13), there 

 will be a fall of potential in it from A to B. If from A and the 



Fio. 14. The rheochord arranged to vary the strength of a current passing through 

 a nerve. It consists of two parallel wires connected by a movable metal slider 8. 

 By moving the slider S to the right the resistance of the rheochord in circuit and 

 therefore the amount of battery current passing through the nerve would be in- 

 creased. 



movable contact S two electrodes pass to a nerve, the current 

 from the battery has two circuits open to it and can pass either 

 through the nerve or along the monochord back to the battery. The 



