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d) Field emission, in which electrons are drawn from the
surface of a metal by the application of very powerful
electric fields.

3. Electrochemistry, Batteries and Other Sources of e.m.f.

Pure liquids are good insulators but liquids containing salts
conduct electricity.
An ion is an atom, which has either lost an electron (a
positive ion) or has gained an electron (a negative ion).
Electrolysis is the process of decomposing an electrolyte by
the passage of electric current through it; this results in chemical
action at the electrodes, that is, the anode and the cathode.
Electrolysis is the basis not only of many forms of chemical
extraction and refining but also of the electroplating industry.
Faraday’s laws describe the laws, which govern electrolysis. An
electrical cell consists of two sets of plates immersed in an
electrolyte. Cell can be either dry or wet. A primary cell cannot be
recharged but a secondary cell can be recharged. A battery is an
interconnected group of cells. All cells have an internal resistance
whose value is reduced by the use of a depolarizes.
Electricity can be produced by a number of different
methods including chemical action, thermoelectricity, the Hall
effect, the piezoelectric effect and the photovoltaic effect.

4. Resistors and Electrical Circuits

A resistor may either be fixed or variable. Variable resistors
may either have a sliding contact or may be “tapped” at various
paints along their length; they may be connected as potentiometers
to provide a variable output voltage. The resistance of a resistor
depends on several factors including the resistively the length, the
cross-sectional area and the temperature of the material. The
conductance of a conductor is equal to the reciprocal of the
resistance. In the case of a conductor an increase in temperature
causes an increase in resistance and vice versa. In an insulator and
a semiconductor, an increase in temperature causes a decrease in

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