128 ELEMENTARY LESSONS ON [CHAP. in. 



These chemical actions stop, and the electricity ceases 

 also to flow, as soon as the wires are separated from one 

 another. They begin when the " circuit " is made; they 

 cease when the circuit is broken. 



153. Effects produced by Current. The cur- 

 rent itself cannot be seen to flow through the wire 

 circuit ; hence to prove that any particular cell or 

 combination produces a current requires a knowledge 

 of some of the effects which currents can produce. These 

 are of various kinds. A current flowing through a thin 

 wire will heat it ; flowing near a magnetic needle it will 

 cause it to turn ; flowing through water and other 

 liquids it decomposes them ; and, lastly, flowing through 

 the living body or any sensitive portion of it, it produces 

 certain sensations. These effects, thermal, magnetic, 

 chemical, and physiological, will be considered in special 

 Lessons. 



154. Voltaic Battery. If a number of such simple 

 cells are united in series, the zinc plate of one joined to 

 the copper plate of the next, and so on, a greater differ- 

 ence of potentials will be produced between the copper 

 " pole " at one end of the series and the zinc " pole " at 

 the other end. Hence, when the two poles are joined 

 by a wire there will be a more powerful flow of electricity 

 than one cell would cause. Such a combination of 

 Voltaic Cells is called a Voltaic Battery. 1 



155. Electromotive -Force. The term "electro- 

 motive-force" is employed to denote that which moves 

 or tends to move electricity from one place to another. 2 



1 By some writers the name Galvanic Battery is given in honour of 

 Galvani ; but the honour is certainly Volta's. The electricity that flows 

 thus in currents is sometimes called Voltaic Electricity^ or Galvanic 

 Electricity , or sometimes even Galvanism (!); but, as we shall sSe, it differs 

 only in degree from Frictional or any other Electricity, and both can flow 

 through wires, and magnetise iron, and decompose chemical compounds. 



2 The beginner must not confuse " Electromotive -force" or that which 

 tends to move electricity, with Electric "force" or that force with 

 which electricity tends to move matter Newton has virtually defined 

 " force," once for all, as that which moves or tends to move matter. When 



