94 PRACTICAL LESSORS IN SCIENCE. 



conductors seem to be necessary to the action which takes 

 place. Do they acquire any new properties? Place a magnetic 

 needle near the tumbler and hold the connecting wires over 

 it, and the needle will be deflected from its north and south 

 direction, which indicates the presence of electricity in the wire. 



This action is explained as follows : the zinc takes the place 

 of the hydrogen of the acid, forming zinc sulphate which re- 

 mains dissolved in the liquid, and the hydrogen is forced 

 along from molecule to molecule through the acid until it 

 reaches the copper plate where it escapes in bubbles. This action 

 is so rapid as to appear continuous, and it is believed to be the 

 cause of the electricity or electrical current in the wire, as the 

 result of the conversion of chemical energy into potential energy. 

 If this action continues the acid becomes weaker, and the zinc is 

 dissolved so that it must be renewed from time to time. 



In order that water may flow from one place to another there 

 must be a difference of level ; so in the case of electricity, there 

 must be a difference in electrical conditions before there can be a 

 flow of electricity. When electricity at any place is greatly in 

 excess of that in the earth it is described as high potential, when 

 slightly in excess as low potential; when there is more electricity 

 in one body than in another again the terms high and low poten- 

 tial are used. Electricity moves from the place of high potential 

 to the place of low potential. Differences of potentials and elec- 

 tromotive force are commonly used in the same sense. The zinc 

 is positive, that is, has a higher potential than the copper, and 

 the electricity passes from the zinc through the liquid to the cop- 

 per in the attempt to restore the equilibrium of the potentials. 

 But as the chemical action is practically continuous the motion 

 toward the copper is continuous, and we have what is called an 

 electrical current. The current passes through the liquid between 

 the metal plates and through the wire connecting them, and to 

 this whole path or course of the current the term circuit is applied. 

 The wires are called the conductors, the end of the wire from the 

 copper plate is called the positive pole and the other the negative 

 pole. Separating the poles is called opening the circuit, bringing 



