ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL WORK 9 



FIG. 9. Diagram of mercury key. 



FIG. 10. Mercury key in a battery circuit. 



FIG. 11. Contact key in a battery circuit. 



FIG. 12. Friction key in a battery circuit ; direct method of use. 



FIG. 13. Diagram of short-circuiting key of du Bois-Reymond. 



