Conductance and inductance

Conductance. An iron wire of the same size and length as a copper one is observed to have a greater resistance than that of the copper wire. At any rate, under the same conditions the copper wire will allow more current to flow than the iron wire does. In other words, copper is better able to conduct electric charges than iron. Copper is, therefore, said to have a greater conductance than iron. Conductance is also called "conductivity", both these terms being synonyms. Conductivity is obviously the opposite of resistance. At least, the greater conductivity a substance has, the less is its resistance.

As, a matter of fact, the term "conductance" means the ability to carry the current; "resistance", on the other hand, is the opposition to the current flow. Whereas the unit of resistance is the ohm, the unit to be used for measuring conductance is the Siemens or the mho which contains the letters o, h, m written in reverse order.

Conductance in Gases. As for conductivity in open air, air is found to be a conductor only when ionized. At any rate, in ordinary daylight at atmospheric pressure air and most other gases act like insulators of small dielectric strength, i. e., they break down and allow a spark to pass under comparatively small potential gradients. At a greater pressure.they are better insulators and less readily allow the spark to pass.

Superconductivity has long been the subject of pure theory and it seemed to be impossible to apply it in practice. In recent years, however, there have been developed instruments using this phenomenon. The study of different substances at low temperatures has discovered many interesting phenomena. One of the most interesting was superconductivity, that is to say, the complete loss of resistance to electric current. This property has been found in more than 20 metals. If an electric current is sent through a ring of cooled metal of this kind, it is expected to circulate for a very long time.

Inductance. We should like you to take into consideration that voltage, resistance and capacity are the three important properties to influence the flow of current in an electric circuit. Besides voltage, resistance and capacity in the circuit, an alternating current is influenced by an additional factor, namely, inductance. That is why we shall turn our attention to inductance here.

A ball has no power by which it can put itself in motion but to throw it means to impart energy to it and this is the reason why it speeds through the air. The ball requires a certain length of time for starting and likewise for stopping. It is this property that one calls inertia.

An electric current acts in that very way, that is to say, it takes time to start and once started it takes time to stop. The factor of the circuit to make it act like that is its inductance.

In any case, inductance is the property which opposes the flow of current as resistance does but in a different manner. By virtue of varying the current which passes through the circuit containing inductance an e. m. f. is induced in this circuit. The e. m. f. known as induced e. m. f. is found to impede any change of current magnitude. The inductance of a circuit is, therefore, of importance only where the current is changing. Hence, one would expect an alternating current to be greatly affected by the presence of an inductance coil in the circuit and such indeed is the case.

It goes without saying that a steady direct current has no inductive effect.