196 ELEMENTARY GENERAL SCIENCE CHAP. 



Voltaic Cells in which Polarisation is prevented. (a) By mechani- 

 cal means. Smee's Cell. A platinised silver plate is substituted for 

 the copper one. Owing to the roughness of such a plate the hydrogen 

 bubbles stream off it. 



(b) By chemical means. Daniell's Cell. The hydrogen, instead of 

 being allowed to collect, is made to act upon a solution of copper 

 sulphate which it decomposes, forming sulphuric acid with it and 

 liberating metallic copper. This copper being deposited on the 

 copper plate does not interfere with the passage of the current. 



Bunsen's and Grove's Cells. Here the hydrogen is got rid of by 

 oxidation. This is effected by causing it to come in contact with 

 strong nitric acid, which being rich in oxygen, changes the hydrogen 

 into water, or oxidises it. The nitric acid is itself reduced, first to 

 nitrous acid and eventually to oxides of nitrogen. In Bunsen's cell 

 a block of gas carbon takes the place of the copper plate of the 

 simple voltaic cell, and in Grove's a plate of platinum. 



Leclanch^'s Cell is the one commonly used in houses for ringing 

 electric bells. The liquid used is a solution of ammonium chloride. 

 The hydrogen formed by the action of the liquid on the zinc plate 

 is got rid of by causing it to combine with oxygen from black oxide 

 of manganese which surrounds the carbon pole. 



Positive and Negative Poles. The negative ( - ) pole is usually of 

 zinc and is the one where chemical action begins and consequently 

 where the electric current starts in the liquid. 



The positive ( + ) pole is the plate where the chemical action is 

 completed and where the current starts outside the liquid. 



QUESTIONS ON CHAPTER XII. 



1. What happens under the following circumstances : 



(a) A piece of commercial zinc is plunged into dilute sulphuric 

 acid. 



(b) Pure zinc is treated as in (a). 



(c) Pieces of amalgamated zinc and ordinary copper are placed 

 parallel to one another in dilute acid and connected by a 

 copper wire outside the liquid. 



2. Before fitting up a simple voltaic cell both plates are accurately 

 weighed. They are weighed again after the current has passed for 

 ten minutes. What differences will be noticed and how would you 

 account for them ? 



3. What properties do you associate with the wire connecting the 

 plates of a simple voltaic cell ? 



4. Explain as fully as you can what is meant by potential. State by 

 the use of this term how it is that an electric current passes round a 

 closed circuit. 



5. It is noticed that after a short time the strength of the electric 

 current from a simple voltaic cell becomes very much diminished. 

 How do you account for this ? 



6. Explain what is meant by the polarisation of a cell. What 

 devices are employed to get rid of polarisation ? 



