CH. X.] 



BATTERIRS AND KEYS. 



solution of copper sulphate. Each metal has a binding screw 

 attached to it, to which wires can be fastened. The zinc rod is 

 called the positive element, the copper the negative element. The 

 distal ends of the wires attached to 

 these are called poles or electrodes, 

 and the pair of electrodes may be 

 conveniently held in a special form 

 of holder. The electrode attached 

 to the positive element (zinc) is 

 called the negative pole or katJiode; 

 that attached to the negative ele- 

 ment (copper) is called the positive 

 pole or anode. If now the two elec- 

 trodes are connected together, an 

 electrical, galvanic or constant cur- 

 rent flows from the copper to the 

 zinc outside the battery, and from 

 the zinc to the copper through the 

 fluids of the battery ; if the elec- 

 trodes are not connected the circle is broken, and no current can 

 flow at all. If now a nerve or muscle is laid across the two 



ig- 133. Diagram of a Daniell's 

 battery. 



electrodes the circuit is completed, 

 and it will be noticed at the moment 

 of completion of the circuit the muscle 

 enters into contraction ; if the muscle 

 is lifted off the electrodes, another 



contraction occurs at the moment the circuit is broken. The same 

 thing is done more conveniently by means of a key : fig. 134 



K.P. I 



Fig. 134. A. Du Bois 

 lleymond's Key. 



