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.
[Illustration: FIGS. 76 to 78.--Various covered wires. fig. 76, single;
fig. 77, duplex; fig. 78, automobile high tension cable.]
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.”
[Illustration: FIGS. 79 to 81.--Standard porcelain insulators. Fig. 79,
tube type; figs. 80 and 81, grooved insulators.]
=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.
[Illustration: FIG. 82.--Standard two wire porcelain cleat.]
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.