Fig. 55.—Wheelock cell; the elements are carbon and zinc. To set up, place the grid in the bottom of the jar and fill its two troughs each about half full of mercury. Place the porous cell in position on the grid so that it sits perfectly upright, resting in the recess of the latter. The zincs stand with lower ends resting in mercury in the troughs of the grid. Into the porous cell, to a height of only two-thirds full, pour solution consisting of equal parts water and sulphuric acid, by measure. Add to this 1⁄2 pound nitrate soda, 1 ounce chromic acid. This solution may be made up in the above proportion and kept in covered receptacle in any desired quantity, ready for use. In the outer jar for 6 × 8 size, 21⁄2 pints of water, and 1⁄2 gill sulphuric acid, 1 part sulphuric acid to 20 parts water, or as much sulphuric acid as it will take without boiling. When a charge becomes exhausted it may be renewed by adding sulphuric acid and salts in the proportions given above, after drawing out with syringe enough of the old solutions to make room for the additions, but the best action is obtained with entirely new solutions. Zincs must be kept thoroughly amalgamated by keeping a good supply of mercury in the troughs.

When a current is produced by a Daniell cell:

1. Copper is deposited on the copper plate;
2. Copper sulphate is consumed;
3. The sulphuric acid remains unchanged in quantity;
4. Zinc sulphate is formed;
5. Zinc is consumed.

If, however, the copper sulphate solution be too weak, the water is decomposed instead of the copper sulphate, and hydrogen is deposited on the copper plate. This deposit of hydrogen lowers the voltage, hence care should be taken to maintain an adequate supply of copper sulphate.

The voltage of a Daniell cell varies from about 1.07 volt to 1.14 volt, according to the density of the copper sulphate solution and the amount of zinc sulphate present in the dilute sulphuric acid.

“Dry” Cells.—It is often necessary to use cells in places where there is considerable jarring or motion, as for automobile or marine ignition. The ordinary cell is not well adapted to this service on account of the liability of spilling the electrolyte, hence, the introduction of the so-called dry cell.

A dry cell is composed of two elements, usually zinc and carbon, and a liquid electrolyte. A zinc cup closed at the bottom and open at the top forms the negative electrode; this is lined with several layers of blotting paper or other absorbing material.

The positive electrode consists of a carbon rod placed in the center of the cup; the space between is filled with carbon—ground coke and dioxide of manganese mixed with an absorbent material. This filling is moistened with a liquid, generally sal-ammoniac. The top of the cell is closed with pitch to prevent leakage and evaporation. A binding post for holding the wire connections is attached to each electrode and each cell is placed in a paper box to protect the zincs of adjacent cells from coming into contact with each other when finally connected together to form a battery.

Points Relating to Dry Cells.—The following instructions on the care and operation of dry cells should be carefully noted and followed to get the best results:

Figs. 56 and 57.—Round and rectangular types of the so-called “dry” cell.
1. In renewing dry cells (or any other kind of cell), a greater number should never be put in series than was originally required to do the work, because the additional cells increase the voltage beyond that required, which causes more current than is necessary to flow through the coil. This increased current flow shortens the life of the battery.
2. In connecting dry cells in places where there is vibration, heavy copper wire should not be used, because vibration will cause it to break.
3. Water should not be allowed to come in contact with the paper covers of the cells because they form the insulation, hence, when moist, current will leak across from one cell to another, resulting in running down the battery.
4. Dry cells will deteriorate when not in use, making it necessary to renew them about every sixty days. The reason dry cells deteriorate is because the moisture evaporates. Freezing, exposure to heat, and vibration which loosens the sealing, causes the evaporation.
5. Weak cells can be strengthened somewhat by removing the paper jacket, punching the metal cup full of small holes, and then placing in a weak solution of sal-ammoniac, allowing the cells to absorb all they will take up. This is only to be recommended in cases of emergency when they are hard to get.
Figs. 58 to 63.—Various zincs; fig. 58 Fuller; fig. 59 Daniell; fig. 60 Leclanche square; fig. 61 Leclanche round; fig. 62 Sampson; fig. 63, bottle.
6. The average voltage of a dry cell when new is one and one-half volts, while the amperage ranges from about twenty-five to fifty amperes according to size.
7. A dry cell when fresh should show from 20 to 25 amperes when tested; the date of manufacture should also be noted as fresh cells are most efficient.
8. Dry cells should be tested with an ammeter, care being taken to do it quickly as the ammeter being of a very low resistance short circuits the cell. A volt meter is not used in testing because, while the cells are not giving out current, their voltage remains practically the same, and a cell that is very weak will show nearly full voltage. When no ammeter is at hand, the battery current may be tested by disconnecting the end of one of the terminal wires and snapping it across the binding post of the other terminal; the intensity of the spark produced will indicate the condition of the battery.

Points Relating to the Care of Cells.—To get the best results from primary cells, they should receive proper attention and be maintained in good condition. The instructions here given should be carefully followed.

Figs. 64 to 66: Various carbons; fig. 61 Cylindrical form; fig. 65 Calland star; fig. 66, wheel.

Cleanliness.—In the care of batteries, cleanliness is essential in order to secure best results. Zincs and coppers should be thoroughly cleaned every time a cell is taken out of use. The zinc, after being thoroughly cleaned, should be rubbed with a little mercury. This prevents local action. Porous cups should be soaked in clean water four or five hours and then wiped dry.

The terminals of each cell should be thoroughly cleansed and scraped bright so as to get good contact of the connecting wires and thus avoid extra resistance in the circuit.

Separating the Elements.—Obviously the positive and negative elements of a cell must not be in contact within the exciting fluid; they should be separated by a space of 3⁄8 to 1⁄2 inch. In the case of cells without porous cups, periodic attention must be given to ensure this condition being maintained.

Figs. 67 to 69.—Various zincs: fig. 67 Crowfoot; fig. 68 Lockwood; fig. 69 fire alarm.

Creeping.—As evaporation of the electrolyte takes place in a cell, it increases in strength, and crystals are left on the sides of the jar previously wetted by the solution, the action being very marked when the solution is a saturated one. The space between these crystals and the side of the jar acts as a number of capillary tubes, and draws up more liquid, which itself evaporates and deposits crystals above the former ones. So that finally the film of crystals passes over the edge of the jar and forms on the outside, thus making a kind of syphon which draws off the liquid. This action may, to a great extent, be prevented by warming the edges of the glass, or stoneware, jars, and of the porous pots, before the cells are made up, and dipping them while warm into some paraffin wax melted in warm oil, a precaution that should always be carried out when a dense solution of zinc sulphate is employed in the cell.

Amalgamated Zinc.—To “amalgamate” a piece of zinc, dip it into dilute sulphuric acid to clean its surface, then rub a little mercury over it by means of a piece of rag tied on to the end of a stick, and lastly, leave the zinc standing for a short time in a dish to catch the surplus mercury as it drains off.

Figs. 70 and 71.—Two forms of copper element: fig. 70, regular form for crowfoot cell; fig. 71, signal pan bottom copper.

The action of the amalgamated zinc is not well understood; by some it is considered that amalgamating the zinc prevents local currents by the amalgam mechanically covering up the impurities on the surface of the zinc and preventing their coming into contact with the liquid. By others it is thought that amalgamating the zinc protects it from local action by causing a film of hydrogen gas to adhere to it. This theory is based on the fact that while no action takes place when amalgamated zinc is placed in dilute sulphuric acid at ordinary atmospheric pressure, the creation of a vacuum above the liquid causes a rapid evolution of hydrogen, which, however, stops on the readmission of the air.

Amalgamating a zinc causes it to act as a somewhat more positive substance than before, therefore the voltage of a cell containing amalgamated zinc is slightly higher than that of a cell constructed with unamalgamated zinc.

Fig. 72.—Diagram of a series battery connection: four cells are shown connected by this method. If the cell voltage be one and one-half volts, the pressure between the (+) and (-) terminals of the battery is equal to the product of the voltage of a single cell multiplied by the number of cells. For four cells it is equal to six volts.

The addition of a very small amount of zinc to mercury causes the mercury to act as if it were zinc alone, arising perhaps from the amalgam having the effect of bringing the zinc to the surface.

Battery Connections.—There are three methods of connecting cells to form a battery; they may be connected:

1. In series;
2. In parallel;
3. In series multiple.

A series connection consists in joining the positive pole of one cell to the negative pole of the other, as shown in fig. 72; this adds the voltage of each cell.

Thus, connecting in series four cells of one and one-half volts each will give a total of six volts.

Fig. 73 illustrates a parallel or multiple connection; this is made by connecting the positive terminal of one cell with the positive terminal of another cell and the negative terminal of the first cell with the negative terminal of the second cell.

Fig. 73.—Diagram of a multiple or parallel connection. When connected in this manner the voltage of the battery is the same as that of a single cell, but the current is equal to the amperage of a single cell multiplied by the number of cells. Thus with 11⁄2 volt 15 ampere dry cells, the combination or battery connected as shown would give 4 × 15 = 60 amperes at a pressure of 11⁄2 volts.
Fig. 74.—Diagram of a series multiple connection. Two sets of cells are connected in series and the two batteries thus formed, connected in parallel. The pressure equals the voltage of one cell, multiplied by the number of cells in one battery, and the amperage, that of one cell multiplied by the number of batteries. This form of connection is objectionable unless all the cells be of equal strength. If old cells be placed on one side and new cells on the other, current will flow (as in fig. 75) from the stronger through the weaker until the pressure of all the cells thus becomes equal. This process therefore wastes some of the energy of the strong cells.

A paralleled or multiple connection adds the amperage of each cell; that is, the amperage of the battery will equal the sum of the amperage of each cell.

For instance, four cells of twenty-five amperes each would give a total of one hundred amperes when connected in parallel.

A series multiple connection, fig. 74, consists of two series sets of cells connected in parallel. In series multiple connections the voltage of each set of cells or battery must be equal, or the batteries will be weakened, hence each battery of a series multiple connection should contain the same number of cells.

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.

Fig. 75.—Diagram to illustrate 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 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
Good conductors
(metals and alloys)[4]		Silver
Copper
Aluminum
Zinc
Brass (according to composition)
Platinum
Iron
Nickel
Tin
Lead
German silver (copper 2 parts, zinc 1, nickel 1)
Platinoid (German silver 49 parts, tungsten 1 part)
Antimony
Mercury
Bismuth.
Fair conductorsCharcoal and coke
Carbon
Plumbago
Acid solutions
Sea water
Saline solutions
Metallic ores
Living vegetable substances
Moist earth.
Partial conductorsWater
The body
Flame
Linen
Cotton
MahoganyDry woods
Pine
Rosewood
Lignum Vitæ }
Teak
Marble.
Insulators, or so-called
non-conductors.		Slate
Oils
Porcelain
Dry leather
Dry paper
Wool
Silk
Sealing wax
Sulphur
Resin
Gutta-percha
Shellac
Ebonite
Mica
Jet
Amber
Paraffin wax
Glass (varies with quality)
Dry air.
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.”

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.

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.

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 quantities is measured by comparison with ascertained standards, the particular methods of comparison varying, however, to meet the circumstances 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 pass 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 passes more freely from one conductor to the other.

The following are the laws of the resistance of conductors:

1. The resistance of a conducting wire is proportional to its length.

If the resistance of a mile of telegraph wire be 13 ohms, that of fifty miles will be 50 × 13 = 650 ohms.

2. The resistance of a conducting wire is inversely proportional to the area of its cross section, and therefore in the usual round wires is inversely proportional to the square of its diameter.

Ordinary telegraph wire is about 1⁄6th of an inch thick; a wire twice as thick would conduct four times as well, having four times the area of cross section; hence an equal length of it would have only 1⁄4th the resistance.

3. The resistance of a conducting wire of given length and thickness depends upon the material of which it is made—that is, upon the specific resistance of the material.

Conductivity.—This is the inverse of resistance. The term expresses the capability of a substance to conduct the electric current.

If the symbol Y represent the conductivity of a substance, and I the current then:

I/Y = its resistance;

and if R represent the resistance of a substance, then

I/R = its conductivity.

Good conductors of heat are also good conductors of electricity.

Specific Conductivity.—The figure which indicates the relation between one substance and another as to their capacity to conduct electricity is called specific or relative conductivity. Taking the specific conductivity of silver as 100, that of pure copper is 96.

The specific resistance of a substance is the reverse of its relative conductivity. The specific resistance of a metal is generally expressed in millionths of an ohm as the resistance of a centimeter cube of that metal between opposite sides.

The following table gives the data for a few metals:

Substance.Specific Resistance
in Microhms.		Specific
Conductivity.
Silver1.609100.
Copper1.64296.
Gold2.15474.
Iron (soft)9.82716.
Lead19.8478.
German Silver21.4707.5
Mercury (liquid)96.1461.6

The specific resistance of copper is therefore:

1.642 / 1,000,000 ohms, or 1.642 microhms.[6]

Divided Circuits.—If a circuit be divided, as in fig. 83, into two branches at A, uniting again at B, the current will also be divided, part flowing through one branch and part through the other.

The relative strength of current in the two branches will be proportional to their conductivities.

This law will hold good for any number of branch resistances connected between A and B. Conductivity is, as shown before, the reciprocal of resistance.

EXAMPLE—If, in fig 83, the resistance of R = 10 ohms, and R′ = 20 ohms, the current through R will be to the current through R′ as 1⁄10 to 1⁄20; or, as 2:1, or, in other words, 2⁄3 of the total current will pass through R and 1⁄3 through R′. The joint resistance of the two branches between A and B will be less than the resistance of either branch singly, because the current has increased facilities for travel. In fact, the joint conductivity will be the sum of the two separate conductivities.

Taking again the resistance of R = 10 ohms and R′ = 20 ohms, the joint conductivity is

1⁄10 + 1⁄20 = 3⁄20

and the joint resistance is equal to the reciprocal[7] of 3⁄20

or 62⁄3
Fig. 83.—Divided circuit with two conductors in parallel.

In most cases the resistance of the different branches will be alike. This simplifies the calculations considerably. Take, for instance, two branches of 100 ohms resistance each and find the joint resistance.

SOLUTION: 1⁄100 + 1⁄100 = 2⁄100; the reciprocal is 100⁄2 = 50 ohms, or, in other words, the joint resistance is one-half of the resistance of a single branch, and each branch, of course, will carry one-half of the total current in amperes.

With three branches of equal resistance, the joint resistance will be 1⁄3; with four branches 1⁄4; with 100 branches 1⁄100 of the resistance of a single branch.

Fig. 84.—Hydraulic analogy for divided circuits. In the system of pipes shown, water flows from A B to C D through the six vertical pipes 1 to 6, the greatest amount going through the one which offers the least resistance. If pipes 1 to 6 all have the same dimensions, equal quantities of water will flow through them. It follows that the resistance which the water encounters diminishes with the increase in the number of pipes between A B and C D. The electrical circuit presents the same conditions: the greater the number of parallel connections (corresponding to the pipes 1 to 6) the less is the resistance encountered by the current.

If, for instance, the resistance of an incandescent lamp hot be 180 ohms, the joint resistance of 100 such lamps connected in multiple is

180⁄100 = 1.8 ohms.

If the electromotive force of the system is to be, say 110 volts, then, according to Ohm’s law, the current for 100 lamps is:

110⁄1.8 = 61.11 amperes.

giving for each lamp a current of

110⁄180 = .61 ampere.

In the case of two branches only, the following rule may be applied also:

Multiply the two resistances and divide the product by their sum.

Written as a formula:

Joint resistance = (R × R′)/(R + R′)

Again, assuming that R = 10 ohms and R′ = 20 ohms:

Joint resistance (10 × 20)/(10 + 20) = 200⁄30 = 62⁄3 ohms.

This rule cannot be employed for more than two branches at a time.

EXAMPLE—A current of 42 amperes flows through three conductors in parallel of 5, 10 and 20 ohms resistance respectively. Find the current in each conductor.

SOLUTION: Joint Conductance = 1⁄5 + 1⁄10 + 1⁄20 = 7⁄20.

Supposing the current to be divided into 7 parts, 4 of these parts would flow in the first conductor 2 in the second and 1 in the third.

The whole current is 42 amperes.

4⁄7 of 42 = 24.
2⁄7 of 42 = 12.
1⁄7 of 42 = 6.
Current in first conductor = 24 amperes.
Current in second conductor = 12 amperes.
Current in first conductor = 6 amperes.

CHAPTER VII

ELECTRICAL AND MECHANICAL ENERGY

The production of electricity is simply a transformation of energy from one form into another, usually mechanical energy is changed into electrical energy and a dynamo is simply a device for effecting the transformation.

Prof. Fessenden truly remarks there are two independent properties of matter—gravity and inertia—and these give two ways of defining force and energy.

It should always be remembered that electricity is something real, although not easily defined. And then, too, while it is not matter and not energy, yet under proper conditions (it having the power of doing work) it is convenient to speak of its performances as electric energy. The following questions and answers, although few in number, may present the subject with clearness.

Ques. What is energy?

Ans. Energy is the capacity for doing work.

Steam under pressure is an example, a spring bent ready to be released is another form, again, water stored in an elevated tank has capacity for doing work. These examples illustrate potential energy, as distinguished from kinetic energy. Potential energy may be defined as energy due to position, and kinetic energy, as energy due to momentum.

Ques. What is matter?

Ans. Matter is anything occupying space, and which prevents other matter occupying the same space at the same time.

Ques. What name is given the smallest quantity of matter which can exist?

Ans. The atom.

An atom means that which cannot be cut, scratched, or changed in form and that cannot be affected by heat or cold or any known force; although inconceivably small, atoms possess a definite size and mass.

Ques. What is a molecule?

Ans. A molecule is composed of two or more atoms.

Ques. What is the behaviour of these minute bodies?

Ans. They are perpetually in motion, vibrating with incredible velocities.

Ques. Why at this point are definitions of energy and of matter most useful?

Ans. Because, as stated, all electric action is an exhibition of energy, and energy must act through matter as its medium.

Ques. What is the difference between electricity and magnetism?[8]

Ans. The ultimate nature of neither is known. There are, however, some differences. To sustain a current of electricity requires energy. To sustain magnetism requires no energy. A current of electricity is always accompanied by a magnetic field of peculiar form. Magnetism alone cannot produce electricity. Electricity can do work; but magnetism cannot in the same sense—and alike with electricity, neither can it exist without contact with matter.

Ques. How is energy transmitted from one part of a material substance to another?

Ans. Gradually and successively. It requires a medium and also time.

Ques. What is the principal use or function in mechanics of electricity?

Ans. It is purely that of transmission. It corresponds to ropes, shafts and fluids as a medium of conveying and translating power or work.

Ques. What is work?

Ans. Work is the overcoming of resistance through a certain distance.

As a quantity of water moving from a higher to a lower level will do work, so also will a quantity of electricity falling through a difference of potential.

Ques. How is work measured?

Ans. In foot pounds.

Ques. What is a foot pound?

Ans. The amount of work done in raising a weight of one pound one foot or the equivalent, overcoming a pressure of one pound through a distance of one foot.

Ques. What is the electrical unit of work?

Ans. The volt-coulomb.

A volt-coulomb of work is performed when one ampere of current flows for one second in a circuit whose resistance is one ohm, when the pressure is one volt.

The Ampere-Hour.—A gallon of water may be drawn from a hydrant in a minute, or in an hour; it is still one gallon. So in electricity, a given amount of the current, say one coulomb, may be obtained in a second or in an hour.

The ampere is the unit rate of flow.

What is called the electric current is simply the relation of any quantity of electricity passed to the time it is passing; that is