254 GENERAL SCIENCE 



negative charge, because of the tendency of the zinc to 

 oxidize and drive a current through the cell to the copper. 

 This means that the electricity will flow to the end of 

 the wire and then stop unless a connection is made for a 

 complete circuit. The starting point of the current is 

 not in the wire but at the zinc plate where chemical 

 action furnishes the energy. As each atom of zinc 

 unites with the acid molecules and liberates hydrogen 

 an electric charge is produced, and the sum of all of 

 these little charges keeps up the potential difference or 

 electromotive force which drives the current through 

 the entire circuit. The current is made up of the sum 

 of the charges carried by the hydrogen atoms to the 

 copper plate. These hydrogen atoms soon collect on 

 the copper plate in such quantity that hydrogen bubbles 

 are formed. As the hydrogen collects on the copper, 

 making it a less negative plate, the current continues to 

 decrease, because the potential difference or electromotive 

 force is not sufficient to drive, it. The cell is said to be 

 polarized when the copper plate is covered with hydrogen 

 bubbles. The polarization of a simple cell takes place 

 so rapidly that the current decreases to almost nothing 

 in a few minutes. 



If hydrogen bubbles escape from the zinc when it is 

 placed in the cell alone or when the external circuit is not 

 closed, it means that local currents are set up on the 

 zinc itself, due to the impurities in the zinc. The impu- 

 rity in the zinc may be iron or some other metal which 

 receives the electric charge from the hydrogen atoms 

 and then the hydrogen is set free and the current flows 

 from the impurity in the zinc to the pure zinc, making 

 short circuits. This local action can be prevented by 

 dipping the entire strip of zinc in dilute sulphuric acid and 



