168 rUYSIOLOGY OF MUSCLES AND NERVES. 



on a magnetic needle. A piece of zinc and a piece of 

 copper are immersed in diluted sulphuric acid, their 

 projecting edges being connected by a piece of wire. 

 When in this condition the apparatus is said to be closed. 

 Within it circulates a current which passes within the 

 wire from the copper to the zinc, and within the fluid 

 from the zinc to the copper. If the closing wire is 

 observed by itself, no current arises in it until it is 

 joined to the apparatus. And if the apparatus is ob- 

 served by itself, that is, without the closing wire, there 

 is no current present in it. It is only in a closed circle 

 that a current can be generated. It is, however, in the 

 apparatus that the cause which under favourable cir- 

 cumstances gives rise to the electric current, lies ; for if 

 the wire by itself is bent into a circle no current is 

 generated within it. Even the cause of the generation 

 of currents within the apparatus may be shown. If when 

 the apparatus is open, that is, when the circuit is not 

 completed by the addition of the connecting wire, the 

 projecting edges of the copper and zinc are connected 

 with an electrometer, the gold leaflets are seen to di- 

 verge, thus showing that an electric tension prevails 

 at these metallic ends projecting from the fluid. This 

 tension is positive at the copper end, negative at 

 the zinc end. On connecting the two metals by a 

 closing wire, the opposed electric currents unite, and 

 this is the cause of the current in the wire. The force 

 which within the wire exhibited electric tension con- 

 tinues to act, and causes the current to continue to 

 traverse the wire. This is called the electromotive force 

 of the apparatus. It expresses itself, when the apparatus 

 is not closed, in the electric tension at the projecting 

 metallic ends or poles of the apparatus ; and when the 



