Page 68 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Polarisation
When a primary cell delivers current to an external circuit, chemical reactions occur inside the cell, and the
products of the reaction accumulate around the internal electrodes (anode and cathode) of the cell. Very often
one of these products is hydrogen, which accumulates around the cathode (positive terminal). Being a gas, this
is an insulator, and so its presence increases the internal resistance of the cell.
Prevention of polarisation
• Permit the hydrogen to escape to air
• Depolariser like manganese dioxide is added

• Use material that will absorb the hydrogen (Calcium)
Local action
This is caused by the impurities present in zinc rod. When the zinc rod is immersed in acid, the zinc atoms and
the impurity atoms form a large number of local cells and the zinc rod gets consumed even when the cell is not
in use. This defect can be avoided by using amalgamated zinc rods. Mercury dissolves zinc and allows only zinc
atoms to come into contact with acid and thus avoids formation of local cells
Prevention of local action
• Use pure zinc
• Use amalgamated zinc (mercury coated zinc)
Different types of cells and batteries

4.5-volt (3R12) battery, a D Cell, a C cell, an AA cell, an AAA cell, an AAAA cell, an A23 battery, a 9-volt PP3
battery, and a pair of button cells (CR2032 and LR44)

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CITS : Power - Electrician & Wireman - Lesson 13-19 CITS : Power - Electrician & Wireman - Lesson 13-19