PRACTICAL PHYSIOLOGY 



Keys are instruments for making or breaking electrical circuits and 

 for short-circuiting currents. 



The Mercury Key consists of a small cup hollowed out of a piece of 

 vulcanite (Fig. 2). From the cup, which is nearly filled with clean 

 mercury, pass in opposite directions two stout copper wires with 



FIG. 2. The mercury key. 



FIG. 3. The spring key. 



binding-screws ; one wire and binding-screw are fixed to the vulcanite 

 base, the other wire can be raised out of or lowered into the mercury 

 by an insulated handle. In some forms of mercury key the wires 

 connecting the binding-screws to the mercury cup run through the 

 vulcanite ; the ends of these wires are liable to become oxidised and 



dirty, and in consequence they make 

 bad contact with the mercury. In 

 order to avoid this it is only necessary 

 to fix the insulated wires from the 

 battery to the binding-screws and to 

 turn the naked ends of these wires 

 over into the mercury. 



The Spring Key is made of a block 

 of lacquered wood, to one end of which 

 is attached a broad brass spring with 

 a binding-screw, and co the other end 

 a plate of brass with a binding-screw 

 (Fig. 3). When the spring is depressed 

 by the finger its free end touches the 

 brass plate and connects together the 

 two binding-screws. The brass plate 

 carries a clip which can clamp the 

 spring in contact with the plate. 

 The Du Bois Key consists of two metal blocks each carrying two 

 binding-screws and attached to a vulcanite base (Fig. 4). The 



FIG. 4. The Du Bois key. 



