FOOTNOTES:[21]Should any doubt whether these conditions of dual existence are a reality (a doubt, however, which the next case dealt with in the text should remove), we would remind them that a similar difficulty unmistakably existed in the case of Eng and Chang, the Siamese twins. It would have been almost impossible to inflict any punishment on one by which the other would not have suffered, and capital punishment inflicted on one would have involved the death of the other.[22]An instance of the sort turns up in Pope's correspondence with Addison, and serves to explain a discrepancy between Tickell's edition of theSpectatorand the original. In No. 253, Addison had remarked that none of the critics had taken notice of a peculiarity in the description of Sisyphus lifting his stone up the hill, which is no sooner carried to the top of it but it immediately tumbles to the bottom. 'This double motion,' says Addison, 'is admirably described in the numbers of those verses. In the four first it is heaved up by several spondees intermixed with proper breathing places, and at last trundles down in a continual line of dactyls.' On this Pope remarks: 'I happened to find the same in Dionysius of Halicarnassus's Treatise, who treats very largely upon these verses. I know you will think fit to soften your expression, when you see the passage, which you must needs have read, though it be since slipt out of your memory.' These words, by the way, were the last (except 'I am, with the utmost esteem, &c.') ever addressed by Pope to Addison. It was in this letter that Pope with sly malice asked Addison to look over the first two books of his (Pope's) translation of Homer.
FOOTNOTES:
[21]Should any doubt whether these conditions of dual existence are a reality (a doubt, however, which the next case dealt with in the text should remove), we would remind them that a similar difficulty unmistakably existed in the case of Eng and Chang, the Siamese twins. It would have been almost impossible to inflict any punishment on one by which the other would not have suffered, and capital punishment inflicted on one would have involved the death of the other.
[21]Should any doubt whether these conditions of dual existence are a reality (a doubt, however, which the next case dealt with in the text should remove), we would remind them that a similar difficulty unmistakably existed in the case of Eng and Chang, the Siamese twins. It would have been almost impossible to inflict any punishment on one by which the other would not have suffered, and capital punishment inflicted on one would have involved the death of the other.
[22]An instance of the sort turns up in Pope's correspondence with Addison, and serves to explain a discrepancy between Tickell's edition of theSpectatorand the original. In No. 253, Addison had remarked that none of the critics had taken notice of a peculiarity in the description of Sisyphus lifting his stone up the hill, which is no sooner carried to the top of it but it immediately tumbles to the bottom. 'This double motion,' says Addison, 'is admirably described in the numbers of those verses. In the four first it is heaved up by several spondees intermixed with proper breathing places, and at last trundles down in a continual line of dactyls.' On this Pope remarks: 'I happened to find the same in Dionysius of Halicarnassus's Treatise, who treats very largely upon these verses. I know you will think fit to soften your expression, when you see the passage, which you must needs have read, though it be since slipt out of your memory.' These words, by the way, were the last (except 'I am, with the utmost esteem, &c.') ever addressed by Pope to Addison. It was in this letter that Pope with sly malice asked Addison to look over the first two books of his (Pope's) translation of Homer.
[22]An instance of the sort turns up in Pope's correspondence with Addison, and serves to explain a discrepancy between Tickell's edition of theSpectatorand the original. In No. 253, Addison had remarked that none of the critics had taken notice of a peculiarity in the description of Sisyphus lifting his stone up the hill, which is no sooner carried to the top of it but it immediately tumbles to the bottom. 'This double motion,' says Addison, 'is admirably described in the numbers of those verses. In the four first it is heaved up by several spondees intermixed with proper breathing places, and at last trundles down in a continual line of dactyls.' On this Pope remarks: 'I happened to find the same in Dionysius of Halicarnassus's Treatise, who treats very largely upon these verses. I know you will think fit to soften your expression, when you see the passage, which you must needs have read, though it be since slipt out of your memory.' These words, by the way, were the last (except 'I am, with the utmost esteem, &c.') ever addressed by Pope to Addison. It was in this letter that Pope with sly malice asked Addison to look over the first two books of his (Pope's) translation of Homer.
ELECTRIC LIGHTING.
Although we certainly have no reason to complain of the infrequency of attempts in newspapers, &c., as well as in scientific journals, to explain the principles on which electric lighting depends, it does not seem that very clear ideas are entertained on this subject by unscientific persons. Nor is this, perhaps, to be wondered at, when we observe that in nearly all the explanations which have appeared, technical expressions are quite freely used, while those matters about which the general reader especially desires information are passed over as points with which every one is familiar. Now, without going quite so far as to say that there is no exaggeration in the picture presented some time back inPunch, of one who asked whether the electric fluid was 'anything like beer, for instance,' I may confidently assert that the very vaguest notions are entertained by nine-tenths of those who hear about the electric light, respecting the nature of electricity. Of course, I am not here referring to the doubts and difficulties of electricians on this subject. It is well known that Faraday, after a life of research into electrical phenomena, said that when he had studied electricity for a few years he thought he understood much, but when he had nearly finished his observational work he found he knew nothing. In the sense in which Faraday spoke, the most advanced students of science must admit that they know nothing about electricity. But the greater number of those who read about the electric light are not familiar even with electrical phenomena, as distinguished from the interpretation of such phenomena. I am satisfied that there is no exaggeration in a passage which appeared recently in the 'Table Talk' of theGentleman's Magazine, describing an account of the electric light as obtained from some new kind of gas, carried in pipes from central reservoirs, and chiefly differing from common gas in this, that the heat resulting from its consumption melted ordinary burners, so that only burners of carbon or platinum could be safely employed.
I do not propose here to discuss, or even to describe (in the proper sense of the word) the various methods of electric lighting which have been either used or suggested. What I wish to do is to give a simple explanation of the general principles on which illumination by electricity depends, and to consider the advantages which this method of illumination appears to promise or possess.
Novel as the idea of using electricity for illuminating large spaces may appear to many, we have all of us been long familiar with the fact that electricity is capable of replacing the darkness of night by the light of broad day over areas far larger than those which our electricians hope to illuminate. The lightning flash makes in an instant every object visible on the darkest night, not only in the open air, but in the interior of carefully darkened rooms. Nay, even if the shutters of a room are carefully closed and the room strongly illuminated, the lightning flash can yet be clearly recognised. And it must be remembered that though the suddenness of the flash makes us the more readily perceive it (under such circumstances, for instance), yet its short duration diminishes its apparent intensity. This may appear a contradiction in terms, but is not so in reality. The perception that there has been a sudden lighting up of the sky or of a room, is distinct from the recognition of the actual intensity of the illumination thus momentarily produced. Now it is quite certain that the eye cannot assign less than a twenty-fifth of a second or so to the duration of the lightning flash, for, as Newton long since showed, the retinaretains the sensation of light for at least this interval after the light has disappeared. It is equally certain, from Wheatstone's experiments, that the lightning flash does not actually endure for the 100,000th part of a second. Adopting this last number, though it falls far short of the truth—the actual duration being probably less than 1,000,000th of a second—we see that so far as the eye is concerned, an amount of light which was really emitted during the 100,000th part of a second is by the eye judged to have been emitted during an interval 4,000 times as long. It is certain, then, that the eye's estimate of the intensity of the illumination resulting from a lightning flash is far short of the truth. This is equally true even in those cases where lightning is said to be for awhile continuous. If the flashes for a time succeed each other at less intervals than a twenty-fifth of a second, the illumination will appear continuous. But it is not really so. To be so, the flashes should succeed each other at the rate of at least 100,000, and probably of more than 1,000,000 per second.
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 thefall 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 betweenelectricity, 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 canany 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 neutralor 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 weknowis, 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,—orpossiblyin 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 generationof 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 theelectric 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 wiretothe copper of the first battery and the wirefromthe 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 alongany 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, produceeffects 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) 'constitutebetween 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. Platinumhas 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, becauseiron 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.
We have now to consider how light suitable for purposes of illumination may be obtained from the electric current. Hitherto we have considered only light such as might be used for special purposes, where a bright and very intense light was required, where expense and complexity of construction might not be open to special objections, and where in general the absolute steadiness of the light was not an essential point. But those who have seen the electric light used even by the most experienced manipulators for the illustration of lectures will know that the light as so obtained, though of intense brilliancy, is altogether unsuited for purposes of ordinary illumination.
If we consider a few of the methods which have been devised for overcoming the difficulties inherent in the problem of electric lighting, the reader will recognise at once the nature of these difficulties, and the probability of their being effectually overcome in the future, for though much has been done, much yet remains to be done in mastering them.
Let us consider first the Jablochkoff candle, the invention of which brought about, in July 1877, the first great fall in the value of gas property.
The Jablochkoff candle consists of two carbons placed side by side (instead of one above the other in a vertical line). Thus placed, with a slight interval between them, thecarbon rods would allow the passage of the electric current at the place of nearest approach, and therefore of least resistance to its passage. A variable and imperfect illumination would result. M. Jablochkoff, however, interposes between the separate carbon rods a slip of plaster of Paris, which is a non-conducting material. The upper points of the carbon rods are thus the only parts at which the current can cross. They are connected by a little bridge of carbon, which is necessary for the starting of the light—just as in the case of the ordinary electric light, the two carbons must, in order to start the light, be brought into contact. When the current flows, the small bridge of carbon connecting the two points is presently consumed, but the arc between the points is still maintained: for the plaster becomes vitrified by the intense heat of the two carbon points on each side, and melts down as the carbons are consumed. If the light is in any way put out, however, a small piece of carbon must be set again, to form a bridge between the carbon points. Throughout the burning of the Jablochkoff candle the fused portion of the insulating layer forms a conducting bridge between the carbon points; and hence there is a considerable loss of electric force (probably about thirty per cent.), which in the ordinary arrangement would increase the intensity of the light. The great advantage of the candle consists in the circumstance that throughout its consumption the carbon ends are at a constant distance from each other without any mechanical or other arrangement being necessary to maintain them in due position.
One point should be noticed here. In the ordinary arrangement of carbon points, the positive carbon, as we have already said, is much more intensely heated, and consumes twice as fast as the negative carbon. Now, if one carbon of the Jablochkoff candle were connected with the positive, and the other with the negative pole of the battery or of a machine, the former side would consume twice as fast as the latter, and the two points would no longer remain at the same horizontal level, which is essential to the properburning of the Jablochkoff candle. By using a machine which produces alternating currents, M. Jablochkoff obviates this difficulty, the carbons being alternately positive and negative (in extremely rapid succession), and therefore consuming at the same rate.
The Jablochkoff candle lasts only about an hour and a half. But four, six, or more candles may be used in the same globe or lantern, and automatic arrangements adopted to cause a fresh candle to be ignited at the moment when its predecessor is burnt out.
In Paris and elsewhere (as in Holborn, for instance), each Jablochkoff lamp is enclosed in an opal glass globe. Mr. Hepworth remarks on this, that in his opinion the use of the opal globe is a mistake, as it shuts off quite 50 per cent. of the light without any corresponding advantage, except the correction of the glare. 'This wasteful disadvantage will no doubt be remedied in the future,' he says, by the use of some less dense medium. 'Mr. Shoolbred states that from a series of careful photometric experiments carried out by the municipal authorities with the Jablochkoff lights, each naked light is found to possess a maximum intensity of 300 candles. With the opal globe this was reduced to 180 candles, showing a loss of 40 per cent., while during the darker periods through which the light passed the light was as low as 90 candles. It may be mentioned here that Mr. Van der Weyde, who has long used the electric light for photographic purposes, has given much attention to the important problem of rendering the electric light available as an illuminator without wasting it, and yet without throwing the rays directly upon the object to be illuminated. The rays are intercepted by an opal disc about four inches in diameter, and the whole body of the rays is gathered up by a concave reflector (lined with a white material), and thrown out in a flood of pure white light, in which the most delicate shades of tint are discernible. He can use any form of electric candle in this way. Only it should be noticed, before the employment of his method isadvocated for street illumination, that there is a difference between the problems which the photographer and the street-lighter have to solve. The Jablochkoff candle, for instance, must be screened on all sides, and even above, when used to illuminate the streets. If its direct light is allowed to escape in any direction, there will be a mischievous and unsightly beam, and from every point along the path of the beam, the intensely bright light of the candle will be directly visible. Again: it is essential that whatever substance is used to screen the light should be dense enough to cause the whole globe to seem uniformly bright or nearly so. The only modification which seems available (when these essential points have been secured) is that the tint of the globe should be such as to correct any colour which the light may be found to have in injurious excess. We may, however, remark that the objection which has been often raised against the colour of the electric light can hardly be just—the injury to the eyes in certain cases arising probably from the strong contrast between the light and the background on which it is projected. For, as to colour, the electric light derived either from the glowing carbon or from incandescent metal is appreciably the same as sunlight.
The Rapieff burner, employed in the 'Times' office, consists of four carbon pencils, arranged thussymbol(except that the two v's are not in the same plane, but in planes at right angles to each other). The spark crosses the space between the points of the v's, and arrangements are made for keeping the two points at the right distance from each other, and also for keeping the ends of the two pencils which form each point in their proper position. If the current is from any cause interrupted, an automatic arrangement is adopted to allow the current to pass to the other lamps in the same circuit. There are six lamps in circuit at the 'Times' office; and M. Rapieff has exhibited as many as ten. The advantages claimed for this light are the following:—'First, its production by any description of dynamo-electric machinewith either alternating or continuous currents; secondly, great diversibility and complete independence of the several lights, and long duration without change of carbons; and lastly, the extreme facility with which any ordinary workman or servant can renew the carbons when necessary, without extinguishing the lights.' The last-named advantage results, it need hardly perhaps be said, from the use of two carbons to form each point. One can be removed, the other remaining to keep the voltaic arc intact until a new carbon has been substituted for its fellow; then it in turn can be replaced by a new carbon, the new carbon already inserted keeping the voltaic arc intact.
The six lamps at the 'Times' office thoroughly illuminate the room, and give light for working the eight Walter presses used in printing the paper. The light has been thus used since the middle of last October, and it is said that other rooms in the building are shortly to be illuminated in the same manner. 'Each lamp is enclosed in an opal globe of about four inches in diameter, and so little heat is given off, that the hand can be placed on the globe without inconvenience, even after the light has been burning for some time.'
In the Wallace lamp there are two horizontal plates of carbon, about nine inches in diameter, instead of mere carbon points. When the current is passing, these carbon plates are separated by a suitable small distance which remains unchanged. The electric arc, being started at the point along the edge of the carbons where there is least resistance to the passage of the current, gradually passes along the edge of the carbons as combustion goes on, changing the position of the place of nearest approach and consequently of least resistance. The light will thus burn for many hours (even for a hundred with large carbon plates), and any number of lights up to ten can be worked from the machine. The objection to the Wallace lamp is, that the light does not remain at one point, but travels along the whole extent of the carbons. It will not be easy to designa glass shade which will be suitable for a light thus changing in position.
The Werdermann regulator is on an entirely new plan; but it has not yet been submitted to the test of practical working outside the laboratory. The positive carbon, which is lowest, ends in a sharp point, which strangely enough retains its figure, while the carbon burns away at the rate of about two inches per hour. The negative carbon is a block having its under side, against which the positive carbon presses, slightly convex. The positive carbon is pressed steadily against the negative by the action of a weight. The increased resistance to the passage of the current, at the sharp point of the positive carbon, generates sufficient heat to produce a powerful light. The light resembles a steadily radiant star, but 'with all its softness and purity of tint, it is so intense, that adjacent gas-flames are thrown on the wall as transparent shadows.' The light will last for fifteen hours without attention, the positive carbon rod being used in lengths of three feet. The carbon block hardly undergoes any change. When the lamp has been burning a long time, a slight depression can be seen at the place where the positive carbon touches it, but by shifting the carbon in its holder this is easily remedied. Mr. Werdermann lately exhibited a row of ten small lamps burning side by side at the same time. 'The two wires from the machine,' says Mr. Hepworth, were carried one on either side of this row of lamps, branch wires being led from them for the service of each lamp. Mr. Werdermann says that his perfected lamps will be furnished with keys, by which the current can be turned on or off, as in the case of gas. We may say in fact, that in the nature of its connections and various arrangements, it ("the Werdermann lamp") most nearly comes up in convenience to the use of gas.'
We do not yet know certainly what arrangement Mr. Edison employs to obtain the light of which so much has been heard. It is asserted that his light is obtained from the incandescence of an alloy of iridium and platinum,which will bear without fusion a heat[26]of 5,000 degrees Fahrenheit. It would be unsafe, however, to assume that this account is trustworthy, or to infer (as we might in the case of almost any other inventor), that such being the nature of his plan, it could lead to no result of practical value. As has been well remarked by a contemporary writer, whatever Edison's invention may be, 'it is certain to be something to command respect, even if it does not quite come up to the glowing accounts which have reached us in advance.'
The following passage from one of these accounts, which appeared in the 'New York Herald,' will be read with interest, and may be accepted as trustworthy so far as it goes. 'The writer last night saw the invention in operation in Mr. Edison's laboratory. The inventor was deep in experimental researches. What he called the apparatus consisted of a small metal stand placed on the table. Surrounding the light was a small glass globe. Near by was a gas jet burning low. The Professor looked up from hiswork, to greet the reporter, and in reply to a request to view the invention, waved his hand towards the light, with the exclamation, "There she is!" The illumination was such as would come from a brilliant gas jet surrounded with ground glass, only that the light was clearer and more brilliant. "Now I extinguish it and light the gas, and you can see the difference," said Mr. Edison, and he touched the spring. Instantly all was darkness. Then he turned on the gas. The difference was quite perceptible. The light from the gas appeared in comparison tinted with yellow. In a moment, however, the eye had become accustomed to it, and the yellowish tint disappeared. Then the Professor turned on the electric light, giving the writer the opportunity of seeing both, side by side. The electric light seemed much softer; a continuous view of it for three minutes did not pain the eye; whereas looking at the gas for the same length of time caused some little pain and confusion of sight. One of the noticeable features of the light, when fully turned on, was that all the colours could be distinguished as readily as by sunlight. "When do you expect to have the invention completed, Mr. Edison?" asked the reporter. "The substance of it is all right now," he answered, putting the apparatus away and turning on the gas. "But there are the usual little details that must be attended to before it goes to the people. For instance, we have got to devise some arrangement for registering a sort of meter, and again, there are several different forms that we are experimenting on now, in order to select the best." "Are the lights to be all of the same degree of brilliancy?" asked the reporter. "All the same!" "Have you come across any serious difficulties in it as yet?" "Well, no," replied the inventor, "and that's what worries me, for in the telephone I found about a thousand;[27]and so in thequadruplex. I worked on both over two years before I overcame them."'
Other methods, as the Sawyer-Man system, and the Brush system, need not at present detain us, as little is certainly known respecting them. In the former it is said that the light is obtained from an incandescent carbon pencil, within a space containing nitrogen and no oxygen, so that there is no combustion. In the latter the carbon points are placed as in the ordinary electric lamp, but are so suspended in the clasp of a regulator, that they burn 14 inches of carbon without adjustment, the carbons lasting eight hours, and producing a flood of intense white light, estimated as equivalent to 3,000 candles.
I have little space to consider the cost of electric lighting, even if the question were one which could be suitably dealt with in these pages. Opinions are very much divided as to the relative cost of lighting by gas and by electricity; but the balance of opinion seem to be in favour of the belief that in America and France certainly, and probably in this country, where gas is cheap, electric lighting will on the whole be as cheap as lighting by gas. It should be noticed, in making a comparison between this country and others in which coal is dearer, that the cheapness of coal here, though favourable in the main to gas illumination, is also favourable, though in less degree (relatively) to electric lighting. Machines for generating electricity can be workedmore cheaply here than in America. Nay, it has even been found advantageous in some cases to use a gas engine to generate electricity. Thus Mr. Van der Weyde used an Otto gas engine driven at the cost of 6d.an hour for gas, to produce the light which he exhibited publicly on the night of November 9. So that the cheapness of gas may make the electric light cheaper. Then it is to be remembered that important though the question of cost is, it is far from being all-important. The advantages of electric lighting for many purposes, as in public libraries, in cases where many persons work together under conditions rendering the vitiation of the air by gas lighting exceedingly mischievous, and in cases where the recognition of delicate differences of tint or texture is essential, must far more than compensate for some slight difference in cost. The possibility (shown by actual experience to be real) of employing natural sources of power to drive machines for generating electricity, is another interesting element of the subject, but could not be properly dealt with save in greater space than this here available.