Self-Induction

An emf can be induced by varying the number of magnetic lines threading through a circuit, the induced current always opposing the change that is occurring, no matter what causes the change of magnetic flux. It may be due to the motion of a magnet or to the change of current in a nearby electrical circuit as in the transformer. The change of magnetic flux may also be due to a change of current in the coil itself, this effect being known as self-induction.

Suppose several hundred feet of wire, in a single loop, to be connected in series with an incandescent lamp, a 115-volt direct current source, and a switch. The switch being closed, the current in the circuit will increase, in a few millionths of a second, to steady value determined by Ohm’s law. Now let this wire be wound on6to an iron rod to form a coil. When the switch is again closed, the current will increase to the same final value as before, but the time required will be several hundredths of a second. In the coil there are hundreds of turns of wire, side by side. The current in each turn causes magnetic lines that thread through the other turns. An increase of current in any loop varies the flux through all the others, the change of flux of magnetic lines generating an emf. This induced emf opposes the change of current.

Self-induction is known to oppose not only the increase of current in a coil but the decrease also. The circuit being opened, the current will not stop instantly. The forward induced emf will cause a spark to appear at the switch.

In order to demonstrate self-induction, connect a large electromagnet and an incandescent lamp, in parallel with each other, through a resistor to a direct-current source. When you close the switch, at first the increasing current through the coils of the electromagnet increases the flux, thus generating an opposing emf. Self-induction impedes the current through the coil. Most of the current flows through the lamp, which glows brightly. The current having become constant, most of the current flows through the lamp, which glows brightly. The current having become constant, most of the current flows through the coils, and the lamp becomes dim. The switch being opened the flux through the coils will decrease rapidly. The induced emf will make the lamp glow brightly for an instant.

Condensers and Dielectric Materials

The dielectric of a condenser is one of the three essential parts. It may be found in solid, liquid, or gaseous form or in combinations of these forms in a given condenser.

The simplest form of a condenser consists of two electrodes or plates separated by air, this representing a condenser having a gaseous dielectric. If this imaginary condenser had the air between the plates replaced by a nonconducting liquid, such as transformer oil, and if the distance between the plates were the same as in the first case, the capacitance would be found to have increased several times on account of the oil having a higher value of dielectric constant than air which is usually taken as l.

The space between the plates being occupied by a solid insulator, a condenser would result, which would be practical, as far as the possibility of constructing it is concerned. It would be found, in this case too, that the capacitance of the condenser was several times larger than when air was the dielectric.

The mechanical construction of either air or liquid dielectric condensers requires the use of certain amount of solid dielectric for holding the two sets of plates.

There are a great many dielectric or insulating materials available from which one may choose. A material which is very good from the electric standpoint is often found to be poor mechanically or vice versa, air being the gas generally used as a dielectric. Compressed air has been used in some high-voltage condensers, compressed nitrogen and carbon oxide being also in use.

Several kinds of oil have been used in condensers, such as castor oil, cottonseed oil, and transformer oil. More recently electrolytic condensers have come into use in radio equipment for use as filters and by-pass condensers where a large capacitance is required and either a d. c. or pulsating d. c. is applied.

Among the solids to meet the requirements as the condenser dialectical are mice, ceramic materials, and paper. Solid insulators used as mechanical supports in condensers include quartz, glass, porcelain, bakelite, mica, amber, hard rubber, etc.