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205 



Practical Physiology 



CHAPTER I. 



Batteries. A voltaic element or cell usually 

 consists of two metals e.g., zinc and copper im- 

 mersed in a fluid such as dilute sulphuric acid, and 

 the changes (movements of ions) which occur under 

 these circumstances in the fluid produce a disturb- 

 ance of electrical equilibrium in the cell which 

 manifests itself as a difference of electrical potential 

 or pressure at the metals. If wires are connected 

 to these it is found that the end of the wire con- 

 nected with the copper or negative metal is 

 charged with positive electricity, and that connected 

 with the zinc or positive metal is charged with 

 negative electricity ; these ends are called the pos- 

 itive pole, or anode, and the negative pole, or 

 kathode, respectively. The anode is said to be in 

 a condition of higher potential and the kathode in 

 one of lower potential, and when they are joined 

 electrical equilibrium tends to re-establish itself 

 in the circuit thus closed. It is common to speak 

 of a current as flowing from the anode to the 

 kathode outside the battery and from the zinc to 



