On the various forces of nature and their relations to each other

by Michael Faraday

You see, therefore, how we are able, by using this electric spark, to get, first of all, the light into a very small space. That oil-lamp has a burner 3¾ inches in diameter. Compare the size of the flame with the space occupied by this electric light. Next, compare the intensity of this light with any other. If I take this candle, and place it by the side, I actually seem to put out the candle. We are thus able to get a light which, while it surpasses all others in brilliancy, is at the same time not too large; for I might put this light into an apparatus not larger than a hat, and yet I could count upon the rays being useful. Moreover, when such large burners are used in a lantern, we have to consider whether the bars of the window do not interfere to throw a shadow or otherwise; but with this light there will be no difficulty of that sort, as a single small speculum, no larger than a hat, will send it in any direction we please; and it is wonderful what advantages, by reason of its small bulk, we have in the consideration of the different kinds of apparatus required, reflecting or refracting, irrespective of other reasons for using the electric light. And it is these kind of things which make us decide most earnestly and carefully in favour of the electric light.

Fig. 58.

I am going to shew you the effect that will take place with that large lens, when we throw the oil-lamp out of action, and put the electric light into use. It is astonishing to find how little the eye can compare the relative intensities of two lights. Look at that screen, and try to recollect the amount of light thrown upon it from the 3¾ inch lamp of Fresnel; and, now, when we shift the lens sideways, look at the glorious light arising from that small carbon point (fig. 58)—see how beautifully it shines in the focus of that lens, and throws the rays forward. At present, the electric light is put at just the same distance as the oil light; and therefore, being in the focus of the lens, we have parallel rays which are thrown forward in a perfectly straight line—as you will see by comparing the size of the lens with that of the light thrown on the screen. You will now see how far we can affect this beam of light by increasing or diminishing the distance of the lamp. We are able, by a small adjustment, to get a beam of a large or small angle; and observe what power I have now over it,—for if I want to increase the degrees of divergence, I am limited by the power of light, in the case of the oil-lamp; but, with the electric light, I can make it spread over any width of the horizon by this simple adjustment. These, then, are some of the reasons which make it desirable to employ the electric light.

Fig. 59.

By means of a magnet, and of motion, we can get the same kind of electricity as I have here from the battery; and, under the authority of the Trinity House, Professor Holmes has been occupied in introducing the magneto-electric light in the light-house at the South Foreland; for the voltaic battery has been tried under every conceivable circumstance, and, I take the liberty of saying, it has hitherto proved a decided failure. Here, however, is an instrument wrought only by mechanical motion. The moment we give motion to this soft iron in front of the magnet, we get a spark. It is true, in this apparatus it is very small, but it is sufficient for you to judge of its character. It is the magneto-electric light; and an instrument has been constructed, as there shewn (fig. 59), which represents a number of magnets placed radially upon a wheel—three wheels of magnets and two sets of helices. When the machine, which is worked by a two-horse power engine, is properly set in motion, and the different currents are all brought together, and thrown by Professor Holmes up into the lantern, we have a light equal to the one we have been using this evening. For the last six months the South Foreland has been shining by means of this electric light—beyond all comparison, better than its former light. It has shone into France, and has been seen there and taken notice of by the authorities, who work with beautiful accord with us in all these matters. Never for once during six months has it failed in doing its duty—never once—more than was expected by the inventor. It has shone forth with its own peculiar character, and this even with the old apparatus; for, as yet, no attempt has been made to construct special reflectors or refractors for it, because it is not yet established. I will not tell you that the problem of employing the magneto-electric spark for light-house illumination is quite solved yet, although I desire it should be established most earnestly (for I regard this magnetic spark as one of my own offspring). The thing is not yet decidedly accomplished, and what the considerations of expense and other matters may be, I cannot tell. I am only here to tell you as a philosopher, how far the results have been carried; but I do hope that the authorities will find it a proper thing to carry out in full. If it cannot be introduced at all the light-houses—if it can only be used at one—why, really, it will be an honour to the nation which can originate such an improvement as this—one which must of necessity be followed by other nations.

You may ask, what is the use of this bright light? It would not be useful to us, were it not for the constant changes which are taking place in the atmosphere, which is never pure. Even when we can see the stars clearly on a bright night, it is not a pure atmosphere. The light of a light-house, more than any other, is liable to be dimmed by vapours and fogs; and where we most want this great power, is not in the finest condition of the atmosphere, but when the mariner is in danger—when the sleet and rain are falling, and the fogs arise, and the winds are blowing, and he is nearing coasts where the water is shallow, and abounds with rocks,—then is his time of danger, when he most wants this light. I am going to shew you how, by means of a little steam, I can completely obscure this glorious sun, this electric light which you see. The cloud now obscuring the light on the screen is only such a cloud as you see when sitting in a train on a fine summer’s day. You may observe that the vapour passing out of the funnel casts as deep a shadow on the ground as the black funnel; the very sun itself is extinguished by the steam from the funnel, so that it cannot give any light; and the sun itself, if set in the light-house, would not be able to penetrate such a vapour.

Now, the haze of this cloud of steam is just what we have to overcome, and the electric light is as soon, proportionally, extinguished by an obstruction of this kind as any other light. If we take two lights, one four times the intensity of the other, and we extinguish half of one by a vapour, we extinguish half of the other—and that is a fact which cannot be set aside by any arrangement. But, then, we fall back upon the amount of light which the electric spark does give us in aid of the power of penetrating the fog; for the light of the electric spark shines so far at times, that even before it has arisen above the horizon, twenty-five miles off, it can be seen. This intense light has, therefore, that power which we can take advantage of,—of bearing a great deal of obstruction, before it is entirely obscured by fogs or otherwise.

Taking care that we do not lead our authorities into error by the advice given, we hope that we shall soon be able to recommend the Trinity House, from what has passed, to establish either one or more good electric lights in this country.