_The voltage of a series multiple connection is equal to the voltage of one cell multiplied by the number of cells in one battery, and the amperage is equal to the amperage of one cell multiplied by the number of batteries._

[Ill.u.s.tration: FIG. 75.--Diagram to ill.u.s.trate incorrect wiring. The current pressure of the six cell battery being greater than that of the smaller unit, current will flow from the former through the latter until the pressure of the six cells is equal to that of the four cells.]

Fig. 75 shows an incorrect method of wiring in series multiple connection.

If the circuit be open, the six cells, on account of having more electromotive force than the four cells, will overpower them and cause a current to flow in the direction indicated by the arrows until the pressure of the six cells has dropped to that of the four. This will use up the energy of the six cells, but will not weaken the four cell battery.

This action can be corrected by placing a two-way switch in the circuit at the junction of the two negative terminals so that only one battery can be used at a time.

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 gla.s.s 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 { Bra.s.s (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 { Bis.m.u.th.

{ Charcoal and c.o.ke { 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 Vitae } { Teak { Marble.

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

[Ill.u.s.tration: 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."

[Ill.u.s.tration: 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 const.i.tution of gravity cannot be defined. Electricity, though not a substance, can be a.s.sociated 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 gla.s.s raised to a red heat became a conductor; and that sealing wax, pitch, amber, sh.e.l.lac, sulphur, and wax, became conductors when liquefied by heat.

=Heating Effect of the Current.=--If a current of electricity pa.s.s 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 s.p.a.ce for the free pa.s.sage of the current. But, if this thickness be diminished, or the quant.i.ty of electricity pa.s.sing over it be augmented, or, in general, if the ratio of the electricity to the magnitude of the s.p.a.ce afforded to it be increased, the conductor will be found to undergo an elevation of temperature, which will be greater, the greater the quant.i.ty of the electricity and the less the s.p.a.ce supplied for its pa.s.sage.

[Ill.u.s.tration: 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 gla.s.s, porcelain, etc. No substance has the power of absolutely preventing the pa.s.sage 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 pa.s.sing by the conducting power of the moisture. This circ.u.mstance also shows why it is necessary to dry previously the bodies on which it is desired to develop electricity by friction.

CHAPTER VI

RESISTANCE AND CONDUCTIVITY

_Resistance is that property of a substance that opposes the flow of an electric current through it._

The practical electrician has to measure electrical resistance, electromotive forces, and the capacities of condensers. Each of these several quant.i.ties is measured by comparison with ascertained standards, the particular methods of comparison varying, however, to meet the circ.u.mstances of the case.

Ohm's law states that the strength of a current due to an electromotive force falls off in proportion as the resistance in the circuit increases.

It is therefore possible to compare two resistances with one another by finding out in what proportion each will cause the current of a constant battery to fall off.[5]

_Silver is taken as the standard, with the percentage of 100, and the conductivity of all other metals is expressed in hundredths of the conductivity of silver._

=Conductivity of Metals and Liquids.=--The metals in general, conduct well, hence their resistance is small, but metal wires must not be too thin or too long, or they will resist too much, and permit only a feeble current to pa.s.s through them. The liquids in the battery do not conduct nearly so well as the metals, and different liquids have different resistances. Pure water will hardly conduct at all, unless the voltage be very high.

Salt and saltpetre dissolved in water are good conductors, and so are dilute acids, though strong sulphuric acid is a bad conductor. Gases are bad conductors.

=Effect of Heat.=--Another very important fact concerning the resistance of conductors is that the resistance in general increases with the temperature. While this fact is true regarding metals, it does not apply to non-metals. The resistance of different metals does not increase in the same proportion. Iron at 100 degrees C, has lost 39 per cent. of the conducting power it possessed at zero, while silver loses but 23 per cent.

=Laws of Electrical Resistance.=--Resistances in a circuit may be of two kinds:

1. Resistance of the conductors;

2. Resistance due to imperfect contact.

The latter kind of resistance is affected by pressure, for when the surfaces of two conductors are brought into more intimate contact the current pa.s.ses more freely from one conductor to the other.