xii CONSTRUCTION OF SIMPLE VOLTAIC CELLS 187 



collected have disappeared, and notice that the power of the 

 wire to deflect the magnetic needle is regained. 



What the Experiments Teach. The student should have 

 no difficulty in understanding now that a wire which connects a 

 strip of zinc and a strip of copper, which are dipping into dilute 

 sulphuric acid, has new properties. It is in a different condition 

 from an ordinary piece of such wire. Its new condition is 

 described by saying that an electric current is passing along it. 

 The arrangement is, in fact, a means of developing an electric 

 current, and is called a voltaic cell. 



Moreover, the use of such words as "flow" and "current" 

 will probably suggest previous facts which the reader has learnt. 

 He has seen that a flow of heat takes place from a body of high 

 temperature to one of low temperature when they are placed in 

 contact, and that such flow continues until both bodies are of 

 the same temperature. 



Similarly, there is a flow of water from one vessel to another, 

 which are in connection, if the level of the water in one vessel 

 is higher than in the other. Hence we are face to face with the 

 question, What difference is there between the copper and the 

 zinc which causes the condition of things called the electric 

 current ? The name given to the difference of condition in the 

 copper and zinc which corresponds to temperature and water- 

 level is difference of potential. The electric current continues 

 to flow along the copper wire until the potential of the two 

 plates becomes the same, when it ceases. This is the state of 

 affairs after the bubbles of the gas, hydrogen, have collected 

 on the copper plate. 



The electrical condition of the plates of copper and zinc 

 becomes altered by placing them in the sulphuric acid, but the 

 final state is different in the two cases. The copper plate is at 

 a higher potential than the zinc plate, and, consequently, when 

 the plates are joined outside the liquid by a copper wire there is 

 a flow of the electric current from the copper to the zinc. The 

 difference of potential which causes the flow is maintained by 

 the solution of the zinc in the acid. Or, speaking in terms of 

 energy (Chap. IX.), the work of maintaining the current is per- 

 formed by the solution of the zinc. This is similar to the 

 maintenance of the work done by a steam-engine by the burning 

 of the coal in the furnace. It was for this reason that we found 



