Resistance

In everyday conversation the word "resistance" is generally used to mean anything whatever that tends to oppose motion.

If a tram is running at a uniform speed along straight rails, friction tends to reduce the speed of the tram, opposing its motion. Likewise, resistance tends to reduce the flow of the electric current. The power expended in maintaining the current through resistance is transformed into heat. That is why, heat develops in a metallic conductor, whenever current flows. The amount of heat developed when the current is flowing through the conductor is the measure of the ohmic resistance of the conductor.

When an electric current flows through a resistance, there is a loss of energy as well as a loss of voltage or electric pressure. Both these losses are directly proportional to the amount of resistance.

The larger the diameter of the wire, the smaller the resistance is and, hence, the more current can flow through it.

It is Petroff, our first Russian electrician, that established this relationship between current strength and the cross-sectional area of a conductor.

As a rule, if the length of a conductor is doubled, the resistance is doubled and if its cross-sectional area is doubled, its resistance is halved. For example, if a copper wire about 4 m long has a resistance of one ohm, the resistance of an eight-meter long wire is two ohms. In like manner, if a piece of wire were replaced by another one of the same length but of double cross-sectional area, it would offer half its former resistance. This rule for the variation of the resistance of a uniform conductor directly with its length and inversely with its cross-section is an important one and is true for all common conductors. According to this rule the wire used must have as large a cross-section as possible provided it is desirable to keep resistance as low as possible.

It should be mentioned, however, that the diameter and the length of wire are by no means the only factors that influence its resistance. As is well known, the resistance of a conductor depends not only on its diameter and length but also on the kind of substance it is made of and on its temperature.

It was not until 1821 that the above was first stated by Davy, the eminent English scientist.

Suppose that we measure the resistance of a conductor when it is carrying a small current and then remeasure its resistance when it is carrying a current large enough to make it red-hot. What are the results obtained? The resistance of most conductors proved to be greater in the second case.

Of particular interest is the fact that electrons meet more resistance when the conductor is hot than when it is cold.

Doubtless, there are some exceptions to this general rule of increased resistance with increasing temperature. Let us take carbon as an example. Its resistance does increase unless its temperature rises. It differs from metals in this respect. Glass, likewise, when it is hot, conducts current much better than it would, were it cold. Electrolytes, that is to say, solutions through which a current is flowing, also decrease in resistance provided their temperature is increased.

In power transmission one should use as good a conductor as possible so that little power might be lost in heating the conductors of the transmission line.