Principles of electricity

The science of electricity is based upon observation of those phenomena of attraction and repulsion which are comprehended under the term electrostatics. Statical electricity, so named from a Greek word (statikos), which means “causing to stand (or stay),”—also called frictional electricity—is the electricity of stationary charges caused by rubbing together unlike bodies, such as glass and silk (noted in Chapter II). In such cases equal and opposite charges of electricity are always produced. The term statical electricity applies properly, however, to the electricity of all stationary charges, however produced.

The electricity upon the surface of glass is called positive electricity; that upon rubber, negative electricity. When silk is rubbed upon glass it receives a negative charge from the glass and confers a positive charge upon the silk. Wool or fur rubbed on wax or rubber receives a positive charge in exchange for a negative charge; “equal and opposite charges of electricity are always produced.” A piece of glass and a piece of silk attract one another; two pieces of silk or two pieces of glass or wax repel one another, because a body which is positively charged is attracted by one negatively charged and repelled by one negatively charged, and vice versa. A piece of glass rubbed by a piece of silk, under suitable conditions, attracts any other body with which it has not been in contact. The piece of silk will do likewise. In all these cases, the attraction or repulsion becomes weaker with increase of distance between the attracting and repelling bodies.

A third body which has been in contact with a piece of glass or a piece of silk acquires to some extent the properties of the glass or silk with which the third body has been in contact. And, conversely, the glass or silk with which the third body has been in contact attracts or repels with less force than before. If a hand is drawn over the surface of an object after it has been charged with electricity, the electricity disappears. It has been conducted through the hand and the body to the earth. This phenomenon shows that the human body is a conductor of electricity. But most metals are much better conductors. Moist air and damp wood are rather poor conductors, while dry air, dry wood, porcelain, glass, hard rubber and sealing-wax are non-conductors, or insulators.

The term dielectric is used in preference to insulation when reference is made to the property of transmitting induction—a process quite distinct from ordinary transmission of an electric current. In electrostatic induction, a body electrostatically charged induces in a neighboring conductor a like charge in the parts farthest from the charged body, and an unlike charge in the nearer parts; the repelled like charge being removed by connecting any part of the conductor momentarily with the earth, while the bound unlike charge spreads over the whole surface of the conductor and remains there even when the inducing body is moved away, or its charge neutralized, if the conductor is properly insulated.

Dielectric strength refers to the ability of an insulating material to resist rupture by high voltage, measured by the voltage necessary to effect a disruptive discharge through it. Insulation resistance, on the other hand, refers to the ohmic resistance offered by an insulating material to an impressed voltage, tending to induce a breakage of current through it. The term dielectric is used as a synonym for insulator, in the sense that a charge on one part of a non-conductor is not communicated to any other part. A charge given to a conductor spreads to all parts of the body. A dielectric possesses the property of transmitting electric force by induction but not by conduction. A charge on one part of a non-conductor or dielectric is not communicated to any other part.

Jeans suggests that since the presence of magnetic energy is always associated with charges in motion, whereas electrostatic energy is present when all the charges are at rest relatively to each other, it may be proper to identify electrostatic energy with potential energy, and magnetic energy with kinetic energy[14]—i. e., energy due to motion of particles, rather than to energy of position, as of a coiled spring.

Statical energy is distinguished from “current electricity” by the fact that it accumulates on various bodies—is stored up—and as soon as proper connections are made, it discharges instantly. Statical electricity is used by physicians in electrical treatment of diseases and in X-ray work. Machines have been constructed that will produce very strong charges of statical electricity.

If a sufficiently large charge of electricity accumulates upon an insulated conductor in an electrical machine, it finally discharges itself, passing through the air to the nearest body. A flash of lightning is the result of an overcharge of statical electricity accumulated upon cloud particles, and may pass from cloud to cloud or descend to the earth.[15] Careful drivers of gasoline-tank wagons allow an iron or steel chain to drag on the roadway from a metallic connection, which conducts any surplus “static” to the ground. Failure to provide for such an emergency sometimes results in a terrific explosion with consequent loss of life.

About the beginning of the nineteenth century, the Italian scientist, Alessandro Volta (1745-1827),—and other physicists—discovered what has been called, after Volta, voltaic electricity, a current generated by chemical action between metals and different liquids as arranged in a voltaic battery. The term “volt”—the electromotive force which performs work at the rate of one joule per second (one watt) in producing a current of one ampere—was similarly derived.

It was learned that if two different metals, such as copper and zinc amalgam, are placed in a weak acid solution (such as one part H₂SO₄ to four parts H₂O), and connected by a wire fastened securely to the metals, a current of electricity (about two volts) will pass through the wire. Carbon (a non-metal) and a metal upon which the solution acts chemically may be used instead of two metals. There must be chemical action between the liquid and one metal, or there will be no current. Such a combination constitutes a cell, and two or more cells make a battery. The current starts with the zinc, is conducted by the solution to the copper, and thence by wire back to the zinc, completing a circuit. The zinc constitutes the negative pole (or electrode), the copper or carbon the positive pole (or electrode).

A cell frequently employed, where a weak (about 1.1 volts) but constant electromotive force (“E. M. F.”) is required, is one devised by the English physicist, John D. Daniell (1790-1845). In this cell a copper sulphate solution containing a copper electrode is placed in contact (by means of a porous wall or partition—usually an unglazed porcelain cup) with a zinc sulphate solution containing a zinc electrode. The zinc electrode is negative to the copper. At each electrode there exists a potential difference between solution and electrode.[16] The two electrodes being connected externally by a wire, a current of electricity will flow through the wire from the copper to the zinc, and zinc will dissolve at the anode (positive pole) and copper deposited on the cathode (negative pole). The current in this case, as in the preceding, is said to be produced by voltaic action and the cell is a primary battery. Voltaic action and electrolysis—the process of chemical decomposition (or dissociation of compounds or molecules)—by the action of an electric current produced externally (as by a dynamo) and forced through the cell, are essentially identical phenomena, and obey the same laws.[17]

The familiar dry cell contains no liquid which might be spilled, and is very useful for certain purposes, as in automobiles, and in operating door-bells. It is merely a voltaic cell whose chemical contents are made practically solid (or paste-like) by the use of some absorbent, as gelatine, sawdust, etc. In cells of the Leclanché type, a mixture of plaster of Paris, flour, and sal ammoniac takes the place of the solution which acts chemically upon one of the contained metals. When used up, a dry cell must be replaced by an entirely new cell. Two or more dry cells constitute a dry battery.

We have seen that there are two types of charged bodies, of which charged glass and charged silk are familiar examples. It was Dufay (1699-1739) who discovered that there were two kinds of electricity, one of which he called vitreous (from glass) and the other resinous (from resin—amber). The terms “positive” and “negative” in relation to electricity were first applied by Benjamin Franklin, in 1756. To the electricity of the glass rod Franklin gave the name “positive” and to that of the sealing-wax (or hard rubber, amber, etc.) the name “negative.” These names are now universally in use—though French physicists still speak of vitreous and resinous electricity.

I have spoken also of a positive pole (or electrode) and a negative pole (or electrode). The electrodes constituting the two poles of a current are also called the anode and the cathode, the former being the positive electrode and the latter the negative electrode.[18]

When it was learned that electrical charges could be distinguished by two opposing terms—positive and negative—it was natural to suppose that there were two distinct kinds of electricity, or “fluids.” This was the view taken by the French chemist Dufay. But the German electrician Æpinus (1724-1802), in his great pioneer work, “Tentamen Theoriae Electriciatis et Magnetismi” (An Attempt at a Theory of Electricity and Magnetism—1759), considered the mathematical consequences of the hypothesis of a single fluid, attracting all matter but repelling itself. It soon became apparent, however, that he must assume either the existence of two electrical fluids or the mutual repulsion of material particles. He chose the latter theory. He explained the phenomena of the opposite poles as results of the excess and deficiency of a “magnetic fluid,” which was dislodged and accumulated in the ends of the body, by the repulsion of its own particles, and by the attraction of iron and steel, as in the case of induced electricity.[19]

Æpinus, who was unquestionably one of the greatest physicists of the eighteenth century, devised a method of examining the nature of the electricity at any part of the surface of a body, by which means he was enabled to ascertain its distribution. He found that the distribution was in agreement with the attractions and repulsions which objects exert when they are in the neighborhood—“electrical atmosphere”—of electrified bodies. Today we say that such bodies are electrified by induction.

The Æpinian theory of electricity and of magnetism was modified and presented in a new form (in 1788) by Coulomb, with two fluids instead of one. His first task, before reducing the theory to calculation, was to determine the law of the forces involved—not being satisfied, for example, with Newton’s assumption that the attractive force of magnetism is inversely to the cube of the distance. Mayer in 1760, and Lambert a few years later, had found the law to be that of the inverse square. Coulomb desired experimental confirmation of this law before accepting it as established. This he secured by means of his torsion-balance (about 1784).[20]

It was in pursuance of this investigation that Coulomb brought to light for the first time the fact that the directive magnetic forces which the earth exerts upon a needle is a constant quantity, parallel to the magnetic meridian, and passing through the same point of the needle whatever be its position.

Barlow, who had adopted the two-fluid hypothesis, showed that the magnetic “fluids” were collected at the surface of spheres (of iron), the surface being the only part in which there could be detected any magnetism. He demonstrated that a shell of iron produces the same effect as a solid ball of the same diameter. Poisson’s later analysis (1824) showed that this was a consequent to be expected. Merz has well said that what Laplace did for Newton was done by Poisson (1781-1840) “for Coulomb’s elementary law of electric and magnetic action, and on a still larger scale by Gauss, who worked out the mathematical theory and applied it to the case of the magnetic distribution on the earth’s surface. In England, already before Coulomb’s researches were published, Cavendish had, likewise by a combination of experiment and calculation, established the elementary formulae and properties of electrical phenomena.”[21]

Benjamin Franklin, the first American to gain international renown as a scientist, adopted and developed a “one-fluid theory of electricity.” On this supposition the parts of the fluid repel each other, and the excess in one surface of the glass—for example—repels the fluid from the other surface. The fluid itself was regarded by Franklin as positive, the part of the other (negative electricity) being taken by ordinary matter, the particles of which were supposed to repel each other and attract the positive fluid, just as the particles of the negative fluid did on the two-fluid theory.

On both the two-fluid and the one-fluid theories, as we have seen, the particles of the positive fluid repelled each other by forces varying inversely as the square of the distance between them—as shown by both Æpinus and Coulomb. This is true also of the particles of the negative fluid. The particles of the positive fluid attracted those of the negative fluid. In Franklin’s one-fluid theory it was the ordinary particles of matter which attracted the positive fluid and repelled one another. Both theories from their very nature imply, as Sir J. J. Thomson long ago (1906) pointed out, the idea of action at a distance.

In his very interesting book, “Matter and Energy” (1912), Professor Soddy says: “All electrical phenomena can be explained as well on the one-fluid as on the two-fluid idea, but our ignorance at the present time as to whether there are two kinds of electricity or one is fundamental. Until the question is settled, the hopes that have been entertained that, through the study of electricity, we shall be able to arrive at a philosophical explanation of matter, are likely to prove unfounded.”

Our modern view of electrification bears a close resemblance to the one-fluid theory of Franklin, whether we suppose there is one kind of electricity, or two kinds. At all events, if there be such a separate force, or such units of energy, as “positive” electricity, it has never been isolated, as have been the negative atoms or electrons. Negative electrification is but a collection of these negative corpuscles or unit charges. The particles of the “electric fluid” of Franklin correspond to these electrons.

“Instead of taking, as Franklin did, the electric fluid to be positive electricity, we take it to be negative,” says J. J. Thomson, in his “Corpuscular Theory of Matter” (1906). And “the transference of electrification from one place to another is effected by this motion of corpuscles from the place where there is a gain of positive electrification to the place where there is a gain of negative. A positively electrified body is one that has lost some of its corpuscles.”[22]