Part 27

While the lightning flash shows the brilliancy which the electric illumination can attain, it shows also the intense heat resulting from the electric discharge. This might, indeed, be inferred simply from the brilliancy of the light, since we know that this brilliancy can only be due to the intense heat to which the particles along the track of the electric flash have been raised. But it is shown in a more convincing manner to ordinary apprehension by the effects which the lightning flash produces where--in the common way of speaking--it strikes. The least fusible substances are melted. Effects are produced also which, though at first not seemingly attributable to intense heat, yet in reality can be no otherwise explained. Thus, when the trunk of a tree is torn into fragments by the lightning stroke, though the tree is scorched and blackened, a small amount of heat would account for that particular effect, while the destruction of the tree seems attributable to mechanical causes. It is, indeed, from effects such as these that the idea of the fall of thunderbolts has doubtless had its origin, the notion being that some material substance has struck the body thus shattered or destroyed. In reality, however, such destructive effects are due entirely to the intense heat excited during the passage of the electricity. Thus, in the case of a tree destroyed by lightning, the shattering of boughs and trunk results from the sudden conversion of the moisture of the tree (that is, the moisture present in the substance of the tree) into steam, a change accompanied of course by great and sudden expansion. The tree is as certainly destroyed by the effects of heat as is a boiler which has burst, though in each case the expansive power of steam directly works the mischief.

It is the more useful for our present purpose thus to note at the outset both the illuminating and the heating power of the lightning flash (or rather of the electric discharge), because, as will presently be seen, the electric light, while in all cases depending on intensity of heat, may either be obtained in the form of a series of flashes succeeding each other so quickly as to be to all intents and purposes continuous, or from the incandescence of some suitable substance in the path of the electric current.

Let us now consider briefly the general nature of electrical phenomena, or at least of those electrical phenomena which are related to our present subject.

Formerly, when light was supposed to be a material emanation, and heat was regarded as an actual fluid, electricity was in like manner regarded as some subtle fluid which could be generated or dispersed in various ways. At present, it is safer to speak of electricity as a state or condition of matter. If it were not that some very eminent electricians (and one especially whose eminence as a practical electrician is very great) are said to believe still that there is such a thing as an electric fluid, we should have simply asserted that in the present position of scientific research, with the known velocity at which the so-called electric current flows, and the known relations between electricity, heat, and light, the theory of an electric fluid is altogether untenable. It will suffice, however, under the actual circumstances, to speak simply of electrical properties, without expressing any definite opinion respecting their interpretation.

A certain property, called electricity, is excited in any substance by any cause affecting the condition of the substance, whether that cause be mechanical, chemical, thermal, or otherwise. No change can take place in the physical condition of any body without the generation of a greater or less amount of electricity, although in far the greater number of cases there may be no obvious evidence of the fact, while in many cases no evidence may be obtainable even by the use of the most delicate scientific tests.

I have spoken here of the generation of a greater or less amount of electricity, but in reality it would be more correct to speak simply of a change in the electrical condition of the substance. Electricians speak of positive and negative electricity as though there actually were two distinct forms of this peculiar property of matter. But it may be questioned whether it would not be more correct to speak of electricity as we do of heat. We might speak of cold as negative heat precisely as electricians give the name of negative electricity to a relative deficiency of what they call positive electricity; but in the case of heat and cold it is found more convenient, and is more correct, to speak of different degrees of one and the same quality. The difficulty in the case of electricity is that at present science has no means of deciding whether positive or negative electricity has in reality the better claim to be regarded as absolute electricity. Making comparison between electrical and thermal relations, the process which we call the generation of positive electricity may in reality involve the dispersion of absolute electricity, and so correspond to cooling, not to heating. In this case the generation of what we call negative electricity would in reality be the positive process. However, it is not necessary to discuss this point, nor can any error arise from the use of the ordinary method of expression, so long as we carefully hold in remembrance that it is only employed for convenience, and must not be regarded as scientifically precise.

Electricity may be excited, as I have said, in many ways. With the ordinary electrical machine it is excited by the friction of a glass disc or cylinder against suitable rubbers of leather and silk. The galvanic battery developes electricity by the chemical action of acid solutions on metal plates. We may speak of the electricity generated by a machine as frictional electricity, and of that generated by a galvanic battery as voltaic electricity; in reality, however, these are not different kinds of electricity, but one and the same property developed in different ways. The same also is the case with magnetic electricity, of which I shall presently have much to say: it is electricity produced by means of magnets, but is in no respect different from frictional or voltaic electricity. Of course, however, it will be understood that for special purposes one method of producing electricity may be more advantageously used than another. Just as heat produced by burning coal is more convenient for a number of purposes than heat produced by burning wood, though there is no scientific distinction between coal-produced heat and wood-produced heat, so for some purposes voltaic electricity is more convenient than frictional electricity, though there is no real distinction between them.

Every one knows that when by means of an ordinary electrical machine electricity has been generated in sufficient quantity and under suitable conditions to prevent its dispersion, a spark of intense brilliancy and greater or less length, according to the amount of electricity thus collected, can be obtained when some body, not similarly electrified, is brought near to what is called the conductor of the machine. The old-fashioned explanation, still repeated in many of our books, ran somewhat as follows:--'The positive electricity of the conductor decomposes the neutral or mixed fluid of the body, attracting the negative fluid and repelling the positive. When the tension of the opposite electricities is great enough to overcome the resistance of the air, they re-combine, the spark resulting from the heat generated in the process of their combination.' This explanation is all very well; but it assumes much that is in reality by no means certain, or even likely. All we _know_ is, that whereas before the spark is seen the electrical conditions of the conductor and the object presented to it were different, they are no longer different after the flashing forth of the spark. It is as though a certain line (straight, crooked, or branched) in the air had formed a channel of communication by which electricity had passed, either from the conductor to the object or from the object to the conductor,--or _possibly_ in both directions, two different kinds of electricity existing (before the flash) in the conductor and the object, as the old-fashioned explanation assumes.[23] Again, we know that the passage of electricity along the air-track supposing there really is such a passage, but in any case the observed change in the relative electrical conditions of the conductor and the object, is accompanied by the generation of an intense heat along the aërial track where the spark is seen.

In the case of electricity generated by means of a galvanic battery, we do not note the same phenomena unless the battery is a strong one. We have in such a battery a steady source of electricity, but unless the battery is powerful, the electricity is of low intensity, and not competent to produce the most striking phenomena of frictional electricity. For instance, voltaic electricity, as used in telegraphic communication, is far weaker than that obtained from even a small electrical machine. What is called the positive extremity of the battery neither gives a spark, nor attracts light bodies. The same is true of the other, or negative extremity. The difference of the condition of these extremities can only be ascertained by delicate tests--the deflections of the needle, in fact, by which telegraphic communications are made, may in reality be regarded as the indications of a very delicate electro-cope.

But when the strength of a galvanic battery is sufficiently great, or, in other words, when the total amount of chemical action brought into play to generate electricity is sufficient, we obtain voltaic electricity not only surpassing in intensity what can be obtained from electrical machines, but capable of producing spark after spark in a succession so exceedingly rapid that the light is to all intents and purposes continuous.

Without considering the details of the construction of a galvanic battery, which would occupy more space than can here be spared, and even with fullest explanation would scarcely be intelligible (except to those already familiar with the subject), unless illustrations unsuited to these pages were employed, let us consider what we have in the case of every powerful galvanic battery, on whatever system arranged. We have a series of simple batteries, each consisting of two plates of different metal placed in dilute acid. Whereas, in the case of a simple battery, however, the two different metals are connected together by wires to let the electric current pass (the current ceasing to pass when the wires are disconnected), in a compound battery, in which (let us say) the metals are zinc and copper, the zinc of one battery is connected with the copper of the next, the zinc of this with the copper of another, and so on, the wire _to_ the copper of the first battery and the wire _from_ the zinc of the last battery being free, and forming what are called the poles of the compound battery--the former the positive pole, the later the negative pole.[24] When these free wires are connected, the current of electricity passes, when they are disconnected the current ceases to pass, unless the break between them is such only that the electricity can, as it were, force its way across the gap. When the wires are connected, so that the current flows, it is as though there were a channel for some fluid which flowed readily and easily along the channel. When the circuit is absolutely broken, it is as though such a channel were dammed completely across. If, however, while the poles are not connected by copper wires or by other freely conducting substances, yet the gap is such as the electricity can pass over, the case may be compared to the partial interruption of a channel at some spot where, though the fluid which passes freely along the channel is not able to move so freely; it can yet force its way along, with much disturbance and resistance. Just as at such a part of the course of a liquid stream--say, a river--we find, instead of the quiet flow observed elsewhere, a great noise and tumult, so, where the current of electricity is not able to pass readily we perceive evidence of resistance in the generation of much heat and light (if the resistance is great enough).

It will be observed that I have spoken in the preceding paragraph of the passage of a current along the wire connecting the two poles of a powerful electric battery, or along any substance connecting those poles which possesses the property of being what is called a good conductor of electricity. But the reader is not to assume that there is such a current, or that it is known to flow either from the positive to the negative pole, or from negative to positive pole; or, again, that, as some have suggested, there are two currents which flow simultaneously in opposite directions. We speak conventionally of the current, and for convenience we speak as though some fluid really made its way (when the circuit is complete) from the positive to the negative pole of the compound battery. But the existence of such a current, or of any current at all, is purely hypothetical. I should be disposed, for my own part, to believe that the motion is of the nature of wave-motion, with no actual transference of matter, at least when the circuit is complete. According to this view, where resistance takes place we might conceive that the waves are converted into rollers or breakers, according to the nature of the resistance--actual transference of matter taking place through the action of these changed waves, just as waves which have traversed the free surface of ocean without carrying onward whatever matter may be floating on the surface, cast such matter ashore when, by the resistance of the shoaling bottom or of rocks, they become converted either into rollers or into breakers.

I may also notice, with regard to good conductors and bad conductors of electricity, that they may be compared to substances respectively transparent and opaque for light-waves, or again, to substances which allow heat to pass freely or the reverse. Just as light-waves fail to illuminate a transparent body, and heat-waves fail to warm a body which allows them free passage, so electricity-waves (if electricity really is undulatory, as I imagine) fail to affect any substance along which they travel freely. But as light-waves illuminate an opaque substance, and heat-waves raise the temperature of a substance which impedes their progress, so waves of electricity, when their course is impeded, produce effects which are indicated to us by the resulting heat and light.

A powerful galvanic battery is capable of producing light of intense brilliancy. For this purpose, instead of taking sparks between the two metallic poles, each of these is connected with a piece of carbon (which is nearly as good a conductor as the metal), and the sparks are taken between these two pieces of carbon, usually set so that the one connected with the negative pole is virtually above the one connected with the positive pole, and at a distance of a tenth of an inch from each other or more, according to the strength of the battery. Across this gap between the carbons an arc of light is seen, which in reality results from a series of electric sparks following each other in rapid succession. This arc, called the voltaic arc, is brilliant, but it is not from this arc that the chief part of the light comes. The ends of the carbon become intensely bright, being raised to a white heat. Both the positive and negative carbons are fiercely heated, but the positive is heated most. As (ordinarily) both carbons are thus heated in the open air, combustion necessarily takes place, though it is to be noticed that the lustre of the carbons is not due to combustion, and would remain undiminished if combustion were prevented. The carbons are thus gradually consumed, the positive nearly twice as fast as the negative. If they are left untouched, this process of combustion soon increases the distance between them till it exceeds that which the electricity can pass over. Then the light disappears, the current ceasing to flow. But by bringing the carbon points near to each other (they must, indeed, be made to touch for an instant), the current is made to flow again, and the light is restored.

The following remarks by M.H. Fontaine (translated by Dr. Higgs) may help to explain the nature of the voltaic arc:--'In truth, the voltaic arc is a portion of the electric circuit possessing the properties of all other parts of the same circuit. The molecules swept away from point to point (that is, from one carbon end to the other) 'constitute between these points a mobile chain, more or less conductive, and more or less heated, according to the intensity of the current and the nature and separation of the electrodes' (that is, the quality and distance apart of the carbon or other substances between which the arc is formed). 'These things happen exactly as if the electrodes were united by a metallic wire or carbon rod of small section' (so as to make the resistance to the current great), 'which is but saying that the light produced by the voltaic arc and that obtained by incandescence arise from the same cause--that is, the heating of a resisting substance interposed in the circuit.'

The intensity of the light from the voltaic arc and the carbon points varies with circumstances, but depends chiefly on the amount of electricity generated by the battery. A fair idea of its brilliancy, as compared with all other lights, will be gained from the following statements:--If we represent the brightness of the sun at noon on a clear day as 1,000, the brightness of lime glowing under the intense heat of the oxy-hydrogen flame is about 7; that of the electric light obtained with a battery of 46 elements (Bunsen's), 235. With a battery of 80 elements the brightness is only 238. (These results were obtained in experiments by Fizeau and Foucault.) The intensity does not therefore increase much with the number of the component elements after a certain number is passed. But it increases greatly with the surface, for the experimenters found that with a battery of 46 elements, each composed of 3, with their zinc and copper respectively united to form one element of triple surface, the brightness became 385, or more than one-third of the midday brightness of the sun (that is, the apparent intrinsic lustre of his disc's surface), and 55 times the brightness of the oxy-hydrogen lime-light.

Another way of obtaining an intense heat and light from the electric current generated by a strong battery is to introduce into the electric circuit a substance of small conducting power, and capable of sustaining an intense heat without disintegration, combustion, or melting. Platinum has been used for this purpose. If the conductive power of copper be represented by 100, that of platinum will be represented by 18 only. Thus the resistance experienced by a current in passing through platinum is relatively so great, that if the current is strong the platinum becomes intensely heated, and shines with a brilliant light. A difficulty arises in using this light practically, from the circumstance that when the strength of the current reaches a certain point, the platinum melts, and the circuit being thus broken, the light immediately goes out.

The use of galvanic batteries to generate an electric current strong enough for the production of a brilliant light, is open to several objections, especially on the score of expense. It may, indeed, be safely said that if no other way of obtaining currents of sufficient intensity had ever been devised, the electric light would scarcely have been thought of for purposes of general illumination, however useful in special cases. (In the electric lighting of the New Opera House at Paris, batteries are used.) The discovery by Orsted that an electric current can make iron magnetic, and the series of discoveries by Faraday, in which the relation between magnetism and electricity was explained, made electric lighting practically possible. One of these shows that if a properly insulated wire coil is rapidly rotated in front of a fixed permanent magnet (or of a set of such magnets), currents will be induced in the coil, which may be made to produce either alternating currents or currents in one direction only, in wire conductors. An instrument for generating electric currents in this way, by rapidly rotating a coil in front of a series of powerful permanent magnets fixed symmetrically around it, is called a magneto-electric machine. Another method, now generally preferred, depends on the rotation of a coil in front of an electro-magnet; that is, of a bar of soft iron (bent in horseshoe form), which can be rendered magnetic by the passage of an electric current through a coil surrounding it. The rapid rotation of the coil in front of the soft iron generates a weak current, because iron always has some traces of magnetism in it, especially if it has once been magnetised. This weak current being caused to traverse the coil surrounding the soft iron increases its magnetism, so that somewhat stronger currents are produced in the revolving coil. These carried round the soft iron still further increase its magnetism, and so still further strengthen the current. In this way coil and magnet act and react on each other, until from the small effects due to the initial slight magnetism of the iron, both coil and the magnet become, so to speak, saturated. Machines constructed on this principle are called dynamo-electric machines, because the generation of electricity depends on the dynamical force employed in rapidly rotating the coils.

We need not consider here the various forms which magneto-electric and dynamo-electric machines have received. It is sufficient that the reader should recognise how we obtain an electric current of great intensity in one case from mechanical action and permanent magnetism,[25] and in the other from mechanical action and the mere residue of magnetism always present in iron.

In the cases here considered it is in reality the sudden presentation of the coil (twice at each rotation) before the positive and negative poles of the magnet, which induces a momentary but intense current of electricity. The rotation being exceedingly rapid, these currents succeed each other with sufficient rapidity to be appreciably continuous. A similar principle is involved in the use of what is called the inductive coil, except that in this case the sudden beginning and ceasing of a current in one coil (and not magnetic action) induces a momentary but strong current: matters are so arranged that the current induced by the starting of the inducing current, immediately causes this to cease; while the current induced by the cessation of the inducing current immediately causes this current to begin again: so that by a self-acting process we have a constant series of intense induced currents, succeeding each other with great rapidity, so as to be practically continuous, as with those produced by magneto-electric and dynamo-electric machines.

All that I have said about the voltaic arc, the incandescence resulting from resistance to the current's flow, and so forth, in relation to electricity generated by galvanic batteries, applies to electricity generated by induction coils, or by magneto-electric and by dynamo-electric machines. Only it is to be noticed that in some of these machines the currents alternate in direction with each revolution of the swiftly turning coil, in others the currents are always in the same direction, and in yet others the currents may be made to alternate or not, as may be most convenient.