xii CONSTRUCTION OF SIMPLE VOLTAIC CELLS 195 



EXPT. 190. Dip the free ends of the wires into some very 

 dilute sulphuric acid, contained in a beaker, and notice that 

 bubbles of gas are formed on each wire. 



FlQ. 102. Lines of Force shown by arrangement of Iron Filings upon a Glass 

 Plate lying upon the poles of an Electro-Magnet. 



CHIEF POINTS OF CHAPTER XII. 



Fundamental Facts. (1) Commercial zinc is acted upon by dilute 

 sulphuric acid with evolution of hydrogen. (2) There is no such 

 action when pure zinc or amalgamated zinc is used. (3) If strips of 

 pure or amalgamated zinc and copper be placed in dilute sulphuric 

 acid and the metals connected outside the liquid by a copper wire, 

 the zinc is acted upon and hydrogen bubbles appear on the copper 

 strip. 



Properties of the Connecting Copper Wire. (1) The wire deflects 

 a magnetic needle placed under it. (2) The wire attracts iron filings. 



(3) After a time the magnetic power of the wire becomes feebler. 



(4) If the wires from the copper and zinc plates be dipped into 

 acidulated water, instead of being joined, bubbles of gas appear on 

 the ends of each wire. 



Potential. The electric current passes along the wire because of 

 the difference of potential between its ends. The end of the wire in 

 connection with the copper plate is at a higher potential than 

 that joined to the zinc plate. Potential is analogous to water-level 

 and temperature. 



Electro-motive Force. Since electric currents flow from places of 

 high potential to places of low potential, and the strength of the 

 current is directly proportional to the difference of potential between 

 the places, it is customary to speak of this difference of potential as 

 electro-motive force. Electro-motive force can be directly measured 

 with a Quadrant Electrometer. 



Simple Voltaic Cell. The arrangement of a plate of pure or amal- 

 gamated zinc and one of copper in dilute sulphuric acid constitutes 

 a simple voltaic cell. The hydrogen whicli collects on the copper 

 plate soon stops the flow of the current. This effect is called polari- 

 sation. 



o 2 



