CONDUCTIVITY. SHORT CIRCUITS AND DANGER FROM OVERHEATING IN CIRCUITS

№ 21. CONDUCTIVITY

An iron wire of the same size and length as a copper one has a greater resistance than that of the copper wire. Copper conducts electric charges better than iron. Copper therefore has a greater conductivity than iron. Conductivity is also called “conductance”. Conductivity is the opposite of resistance. If a lamp of one size is placed in one branch of the circuit and a lamp of a different size in another one, the two resistances are unequal. Most of the electric current will certainly follow the path of least resistance, so that the branch containing the smaller resistance will conduct a greater amount of current. That is why it has a greater conductivity. It is clear that the best conductor has the least resistance and the poorest conductor has the greatest resistance.

№ 22. SHORT CIRCUITS AND DANGER FROM OVERHEATING IN CIRCUITS

No electrical appliances and machines are designed for use at very high temperatures. Too much heat damages the insulation. In addition to it this heat is really lost energy. The lines that carry large amounts of current are designed with large cross-section areas so that the heat loss is reduced to a minimum. It is found that overloaded wires will overheat. If the overload is too great, the rubber and cotton insulation burns up and then the circuit is readily shorted. A short circuit is any connection which allows the current to pass through a shorter, less resistance path. A short circuit may be very damaging if a large amount of current is involved. In order to prevent overheating of a circuit we use a protective device by means of which the circuit is broken. The fuse often protects appliances from short circuits. The lines designed to carry large currents have special automatic protection instead of fuses.

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