168 PHYSIOLOGY OF MUSCLES AND NERVES. 



in ; ( magnetic needle. Apiece of zinc and a piece of 

 nipper are immer.-ed in diluted sulphuric acid, their 

 projecting edges being connected by a piece of wire. 

 \VLen in this condition the apparat us 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 

 oh.-erved 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- 

 cuin.-tances gives rise to the electric current, lies ; for if 

 the \\ire by itself is bent into a circle no current is 

 generated within it. Even the cause of the generation 

 of currents within the apparat us 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 po.-,iti\e at the copper end, negative ;( t 

 the /inc end. On connecting the two metals by a 



closing \\ ire, tl ppo>ed electric currents unite, and 



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

 \\hichwithin the wire exhibited electric tension con- 

 tinues to act, and causes the current to continue to 

 tra\erse the \\ire. This is called the electrOTTlOtive force 



of the apparatus It e\pre--;e- itself, when the apparatus 

 i- not dosed, in the electric tension at the projecting 

 metallic ends or pol,- of the apparatus; and \\hen the 



