resistance can be considerable. In fact, as previously mentioned, it is this heat that is made use of in electrical cooking devices. But in an ordinary conductor where it is desired to keep resistance to a minimum, there is energy lost in heat and you may notice that some conductors which are designed to carry very heavy current have asbestos insulation so that the heat generated cannot do any harm.
A large wire can carry heavier current than a small wire. If too heavy a current is passed through a small wire, it will become excessively hot even to the point of melting.
A simple example will bring home to us very forcibly the effect of length of conductors on power loss. A man in New York speaking to a friend in San Francisco might think he is on a single circuit, the conductors of which connect him and the Pacific Coast. If this were the case, it would mean that there would be six thousand miles of wire, and all the resistance in six thousand miles of wire would have to be overcome by the current carrying his voice. But this resistance is so high that if there were only one circuit, the amount of power required to transmit a voice across the country would be so great as to be commercially impossible. How is it done then ? Every fifty miles or so there is a sub-station where the voice is amplified several hundred times, in the same manner as sound signals are amplified in our Radio receiver. Were it not for these amplifying stations and the fact that the voice is amplified hundreds of times on its way across the country, trans-continental telephone service would be an impossibility. At the bottom of all this, the necessity for so much amplification, is the fact that copper, our most commonly used conductor of electrical current, has considerable resistance.
Silver offers slightly less resistance to current flow than copper, but of course it would be impractical to use silver wires in general and Radio work, due to its cost.
From what has been said about copper wire as a conductor of electricity, it is clear that the size of the wire to be used for any particular purpose is important. If made too small, energy will be lost in heat--and if there is too much heat generated to be dissipated rapidly enough, the wire will melt and the circuit will be opened. On the other hand, if we make the wire too large, that is larger than necessary, we are wasting copper. Therefore, the size of wire depends largely on the amount of current it will be called on to carry.
