CHAPTER V

CONDUCTORS AND INSULATORS

Bodies differ from each other in a striking manner in the freedom with which the electric current moves upon them. If the electric current be imparted to a certain portion of the surface of glass or wax, it will be confined strictly to that portion of the surface which originally receives it, by contact with the source of electricity; but if it be in like manner imparted to a portion of the surface of a metallic body, it will instantaneously diffuse itself uniformly over the entire extent of such metallic surface, exactly as water would spread itself uniformly over a level surface on which it is poured.[3]

Bodies in which the electric current moves freely are called _conductors_, and those in which it does not move freely are called _insulators_. There is, however, no substance so good a conductor as to be devoid of resistance, and no substance of such high resistance as to be a _non-conductor_.

_Mention should be made here of the misuse of the word non-conductor; the so-called “non-conductors” are properly termed insulators._

The bodies named in the following series possess conducting power in different degrees in the order in which they stand, the most efficient conductor being first, and the most efficient insulator being last in the list.

=TABLE OF CONDUCTORS AND INSULATORS=

{ Silver { Copper { Aluminum { Zinc { Brass (according to composition) { Platinum { Iron Good conductors[4] { Nickel (metals and alloys) { Tin { Lead { German silver (copper 2 parts, zinc 1, nickel 1) { Platinoid (German silver 49 parts, tungsten 1 part) { Antimony { Mercury { Bismuth.

{ Charcoal and coke { Carbon { Plumbago { Acid solutions Fair conductors { Sea water { Saline solutions { Metallic ores { Living vegetable substances { Moist earth.

{ Water { The body { Flame { Linen { Cotton Partial conductors { Mahogany } { Pine } Dry woods { Rosewood } { Lignum Vitæ } { Teak { Marble.

{ Slate { Oils { Porcelain { Dry leather { Dry paper { Wool { Silk { Sealing wax { Sulphur Insulators, or { Resin so-called { Gutta-percha non-conductors. { Shellac { Ebonite { Mica { Jet { Amber { Paraffin wax { Glass (varies with quality) { _Dry_ air.

The earth is a good conductor; much difficulty is frequently experienced by the wires making contact with some substance that will conduct the electricity to the earth. This is called “grounding.”

=Mode of Transmission.=--The exact nature of electricity is not known, yet the laws governing its action, under various conditions are well understood, just as the laws of gravitation are known, although the constitution of gravity cannot be defined. Electricity, though not a substance, can be associated with matter, and its transmission requires energy. While it is neither a gas nor a liquid, its behavior sometimes is similar to that of a fluid so that it is said to “flow” through a conductor. This expression of flowing does not really mean that there is an actual movement in the wire, similar to the flow of water in a pipe, but is a convenient expression for the phenomena involved.

=Effect of Heat.=--The conducting power of bodies is affected in different ways by their temperature. In the metals it is diminished by elevation of temperature; but in all other bodies, and especially in liquids, it is augmented. Some substances which are insulators in the solid state, become conductors when fused.

Sir H. Davy found that glass raised to a red heat became a conductor; and that sealing wax, pitch, amber, shellac, sulphur, and wax, became conductors when liquefied by heat.

=Heating Effect of the Current.=--If a current of electricity pass over a conductor, no change in the heat condition of the conductor will be observed as long as its transverse section is so considerable as to leave sufficient space for the free passage of the current. But, if this thickness be diminished, or the quantity of electricity passing over it be augmented, or, in general, if the ratio of the electricity to the magnitude of the space afforded to it be increased, the conductor will be found to undergo an elevation of temperature, which will be greater, the greater the quantity of the electricity and the less the space supplied for its passage.

These heat effects are manifested in different degrees in different metals, according to their varying conducting powers.

The poorest conductors, such as platinum and iron, suffer much greater changes of temperature by the same charge than the best conductors, such as gold and copper.

The charge of electricity, which only elevates the temperature of one conductor a small amount, will sometimes render another incandescent, and will vaporize a third.

=Insulators=.--The term insulator is used in two ways: 1, as in insulating substance or medium, and 2, as a specially formed piece of some insulating material, such as glass, porcelain, etc. No substance has the power of absolutely preventing the passage of electric currents between conductors but many have sufficient insulating power for practical purposes. The properties to be desired in a good insulating material are:

1. Permanence; 2. High power of resisting breakdown; 3. Mechanical strength; 4. Fairly high dielectric or insulation resistance; 5. Special qualities for the use to which the material is to be put.

Permanence is the most important quality, and is the one least easily attained. The power of resisting breakdown is a complex quality, for it is not solely dependent on mere puncturing pressure, but also on mechanical goodness, and to a certain extent on the insulation resistance. It cannot be easily determined by a simple laboratory test, but must be found by experience of actual service conditions.

=Impregnating Compounds.=--These are used for the treatment of fibrous materials. They increase the insulating properties of the fibrous materials, render them moisture proof and able to withstand the effect of heat with less rapid deterioration.

When wires or cables are to be used under water, they must be made impervious, and great care must be taken to prevent the water penetrating and thus injuring the insulation.

=Water as a Conductor.=--Water, whether in the liquid or vaporous form, is a conductor, though of an order greatly inferior to the metals. This fact is of great importance in electrical phenomena. The atmosphere contains, suspended in it, always more or less aqueous vapor, the presence of which impairs its insulating property.

The best insulators become less efficient if their surface be moist, the electricity passing by the conducting power of the moisture. This circumstance also shows why it is necessary to dry previously the bodies on which it is desired to develop electricity by friction.