Faraday report to Trinity House   15 August 1854

Report on Electric Light &c

15 Aug 1854 | R. Institution

I beg to report upon the result of the Experimental trials of the Electric light belonging to the Electric power & colour Company which have been recently carried on at the Trinity House¹[.] Dr. Watson has on the part of the Company done all that I desired, and the lamp has been in action at different times for the whole of five days. The trials sufficiently commend the light as most beautiful; and the lamp as one well fitted to bring the Electric forces into action. They also have shewn that the illumination can be sustained for 8 hours or any longer time if needed; and with an intensity much surpassing a Fresnel lamp. They have shewn also the facility of exchanging one lamp for another in less than a second of time:- the power of arranging an intermitting light, and the probable facility of placing 2, 3 or 4 centers of light in a space not more than an inch or an inch & a half wide:- but they have also revealed other circumstances, which would not otherwise have been known, and which I must take carefully into account. For much as I may desire that the Electric light, with its special advantages, may find ultimately its full application in light houses, I am bound before all other things to consider the security and the constancy of the service required, and the necessity that there should be allowed in places apart from society and often very difficult of access inasmuch as such are very frequently the localities of the most important lighthouses; where, if any where, the power of the Electric light would be required. The conclusion I have arrived at may be found at the end of this report; but I give the reasons at some length that they may stand as my justification[.]

The light is sustained by an arrangement of Voltaic batteries. The proposition of Dr. Watson, on the part of the company, is, that as the substances produced by the action of the battery are products of great commercial value, so the company will undertake the arrangement and care of the battery by their own servants, the conveyance of the acids, metals, & other materials, & the removal of the products from the lighthouses; undertaking to supply a constant light and charging a certain price for the light so supplied. Under such an arrangement the following points still remain to claim the attention of the Elder Brethren. A room or covered place would be required for the Voltaic battery, & for the stock of acids metals, cells, &c &c; also living rooms for the man or men in charge of the battery and a free supply of water. The battery employed at the Trinity house was composed of eight dozen pairs; each consisting of amalgamated Zinc & cast iron for the metals, and of strong Nitric acid & somewhat diluted sulphuric acid for the fluids. It occupied a space of 21 feet by 7 feet. Three carboys of oil of vitriol, 16 of Nitric acid, and 14 in the empty state to receive the diluted products, are necessary for a fortnights work of 12 hours to the day; & these with working room would require a space of 14 feet by 14 feet; so that a battery room equivalent to one of 18 or 20 feet square would be necessary even though the supply of materials &c were made regularly every fortnight. If the intervals were longer the place must be larger. The amount of habitation required for the battery men can be judged of from the data connected with the present keepers[.]

I am bound to state that, at the Trinity house, the battery action was accompanied by a serious inconvenience which can be provided against only by removing the building to a distance from the lighthouse[.] It consists in the emission of nitrous fumes, occurring always at the discharge of the battery when the light is extinguished, and often during the course of the general action. It is more liable to occur with acids that have been previously used; nevertheless such must be employed to the utmost for otherwise the required product would not be obtained, there would be great waste of materials and the commercial part of the plan would fail. The battery room requires an open air ventilation; and as it must be a fixture, would be objectionable on any side of a lighthouse, if very near to it. All the persons engaged in the experiments at the Trinity house suffered from these fumes: in the case of one of the workmen it caused spitting of blood, & I consequently interrupted the experiments and made them much shorter than was intended. The place was no doubt close not having a thorough draught, though opening by a large door to the air:- it was the place selected by Dr. Watson[.]

The staff employed about the lamp & battery consisted of three persons; one of superior intelligence having a good comprehension of the principles & practice of the lamp & battery, and two who were labourers of ordinary intelligence. The battery requires a peculiar kind of care[.] There are very numerous screw-clips about it which are conjoined & separated every time the battery is employed; if any one of these fail during the time of action, the Electric current is stopped & the light goes out. Occasionally one or another will become hot & then the power of the light sinks; these must be watched for and cooled with water. From time to time the zinc plates must be amalgamated; but the time is only to be ascertained by watching the action of the plates. About the time when the Nitric acid becomes much charged with iron, a cell here & there will become hot and fume much, & at the same time it will oppose great resistance to the current and the light will diminish; then the connexion has to be made across the cell & the cell itself removed. These uncertain cells have to be watched for;- as many as eight occurred in one days work.

The existence of a battery staff separate from the keepers of the lighthouse appears to me a source of great risk in the constancy & character of the light. The light keepers would depend entirely upon the companys men for the supply and amount of light; for the power of producing & sustaining it would be out of their hands. On the other side the battery men would be obliged to attend to the interests of their employers; they must use the material supplied economically, and exhaust the Nitric acid to the full degree; which as I have stated tends to cause uncertain diminution of the light:- and in this way it is probable that conflicting interests might be involved and at the very same time the responsibility divided and rendered uncertain[.]

If it were proposed that the Trinity House should take the whole matter into their hands, then I need merely point out in the first place that the knowledge & intelligence required by the men in charge must be peculiar in its kind & much above that now needed, and that care & attention would be required to many more points than at present and would need to be unremitting:- and in the second that the Trinity House must become a manufacturing and Commercial body; in respect of which case I must observe that the light could not be obtained at any permissible price if the products of the action were discarded and thrown away; and if they were preserved to be sold for commercial purposes then in my opinion it is not likely that a price could be obtained for them which would be at all proportionate to the first expence of the materials the cost of the labour of using them & their conveyance to the lighthouses and back again.

Next in order to the battery are the connexions consisting of insulated copper wire leading to the lamp[.] These were (in the trial experiments) 250 feet long so that that distance has been practically proved[.] Their nature is simple but as everything depends upon their continuity it must be thoroughly understood by the keepers so that in case of accidents they may know how to search for failures & their places & rectify them[.]

The lamp is an arrangement of supports and adjustments intended to carry two insulated vertical cylinders or rods of hard carbon one over the other between the ends of which the light is produced. Though supplied by the company it would of necessity be under the charge of the light keepers and therefore I am bound to notice the liabilities & possible casualties. It is intricate as compared with the ordinary oil lamp, and the principles concerned in its construction and use are very different; being both more numerous & more refined. In the oil lamp the place of the light is constant, being determined by the fixed burner: in the Voltaic lamp it would change continually if left to itself, for as the lower carbon burns away the place of the light sinks & has to be raised very frequently by hand, or else continuously by clock work:- on the other hand as the upper carbon burns away it requires continual regulation in relation to the lower one; this is effected cleverly in the lamps by an electro magnetic arrangement beneath; but that involves the use of an electro magnet which is liable to change in force; of insulations of the carrying parts, and of mechanical arrangement of holders, levers, and screws which are sometimes liable to fail (failing from time to time in the actual experiments) and which require considerable & peculiar intelligence to understand & keep in order.

The lamp was left at various periods and at one time for 5 hours together; it was found at the end of that time burning brightly & had done so during the intervening time. Nevertheless it went out occasionally (and of course suddenly), sometimes by contact of the carbons and once by a loose screw. Dr. Watson proposes an Electro magnetic alarm which shall give notice if the light is extinguished. This can no doubt be applied but involves another object for intelligent care[.]

The Carbon cylinders, which are essential, are variable in their quality and the proportion of light varies much with them when the same amount of battery force is employed. The lower one burns away at the average rate of 7 inches in 12 hours and the upper one at the rate of 14 inches. They have to be replaced nightly or it may be twice in a night of more than 10 or 12 hours[.] They require careful centering & fixing in the apparatus. The Trinity house would be dependant upon others i.e the company, both for the supply and the quality.

I arrive now at the consideration of the light itself. It flickers much partly because the electric discharge is sometimes with flame & sometimes without, and partly because of variations in the places on the carbon through which the discharge occurs. This is so considerable that the light in a given direction will vary as 1 to 2 or even in higher rates as 1 to 4 or 5. When these flickerings are at short intervals as for instance less than a second then the effect is nearly lost upon an eye at a distance and an average degree of brightness is perceived. But there are other variations, which, occupying the time of three or four seconds or even a longer period, are more important. These depend chiefly upon the continually varying interval between the carbons, which having been adjusted by the Electromagnet, then goes on increasing to a certain extent, when it is again adjusted;- but also upon the quality of the carbons;- upon the general condition of the voltaic battery;- often upon the peculiar condition of a single cell;- and upon the actions of the men at the battery itself. They were occasionally of such degree and in such an interval of time as might I think in bad weather and under hasty observation lead a mariner to mistake a light intended to be fixed for a variable light.

The light of the two lamps used conjointly on the first day & of the single lamps employed on other days, urged in both cases by the force of 96 pairs of plates was always much above that of the great central three wicked oil lamp;- the average result was that it equalled 4.43 such oil lamps. As to the degree of variation caused by the circumstances already referred to, I found it very considerable, especially with a battery charged with second day acid, in which case individual cells were liable to be heated. On such occasions it has varied in the course of 10 minutes in the ratio of 4 to 1 nearly being at one time equal to 5.92 or almost 6 Fresnel lamps, and at another only to 1.57 or 1 1/2 of such standards. Over this variation the keeper of the light has no power & he cannot correct it by adjustment at the lamp.

The source of light is very compressed & intense; it is like a small sun instead of a diffuse & large lamp flame. Hence some supposed theoretical advantages but also some serious practical disadvantages. In the first place it cannot be counted upon as a center of light sending out rays of equal intensity in all directions. Such a result in a horizontal plane is indeed true, as regards the average light; but not in a vertical plane for then the light diminishes rapidly upwards and downwards from a plane of maximum intensity, because of the shadow cast in those directions by the carbon terminals. Another curious point is that the most intense light is not thrown in a horizontal direction. The electric force excavates the Positive carbon into a cup at its extremity whilst the negative carbon becomes conical at the end. As the positive carbon consumes most rapidly the upper is made to assume that condition:- hence the maximum light issues not in a horizontal plane but in a cone which is bent downward to such an extent that the maximum beam forms an angle of 17˚ to 20˚ with the horizontal line.

The compressed condition of the source of light causes it to throw great shadows from opaque objects. The lamp has two upright supporting pillars, about an inch in diameter and 5 or 6 inches from the light on opposite sides these throw a vertical shadow of 7 inches wide at 38 inches from the lamp, this subtending an angle of 10˚ or 11˚:- they would of course eclipse the light to that amount to ships at sea. Dr. Watson proposes to alter the construction & to use flat bar supports instead of cylinders which would do away with much of this shadow. In like manner the upright bars of the window and other opaque objects in a lanthorn would cast shadows, and if of greater width than the light itself, the shadows would increase in width outwards & cause eclipses of the light. The reason why this does not happen sensibly or in the same degree with the Fresnel lamp is that there the light has a horizontal width of nearly 4 inches and so the absolute shadows vanish at a small distance. Dr. Watson proposes to have two or three electric lights arranged in the same horizontal plane; this would involve the use of two or three pairs of carbon conductors; the occurrence of shadows and loss of light from the extra carbons introduced; the necessity of a more complicated lamp;- and also the use of separate batteries for each light, for experience leads to the conviction that such a separate arrangement is required.

Finally the Electric light of one lamp was placed in the center of the catadioptric apparatus. The effect was very beautiful but inconvenient circumstances dependant upon the concentration of the light occurred. The apparatus is intended to gather in the rays through 120˚ or more in a vertical plane and cast them horizontally; and, as in the oil lamp arrangement, the rays are sent forth far more freely in directions above the horizontal plane than below it, (where the shadow of the burner interferes,) so the system of reflectors above the dioptric pannel or first class refractor is more important and more extensive than that below, & in the construction is attended to accordingly. But with the electric light the reverse of this is the case: it is the rays below which are most abundant, those above been cut off to a great extent by the cupped carbon as already described: and not merely so but even the maximum ray is depressed by 18˚ or 20˚, so that much of the intention of the present catadioptric apparatus is rendered nugatory by the conductor of the Electric lamp. The reversion of the current would correct this in part, but that would involve a rapid consumption of the lower carbon; and it may be, give rise to other circumstances of which I have practically no knowledge; Dr. Watson having always made the upper carbon positive.

Neglecting the reflectors I will call attention to the action of the 1st class reflector which would remain the same in whatever way the electric battery was connected[.] The beams which proceed outwards from it are directed horizontally and as a general mass very well so but they are divided into horizontal planes which when they arrive at the screen produce bands, some luminous, some dark, and some most beautiful & intense in colour. The luminous and dark bands are produced by imperfections in the form of the separate prisms of glass constituting the pannel, or by the want of absolute perfection in their association together. Such defects are of no consequence when a flame 1 1/2 inches in height or more is in the focus; for then the rays from the various parts of the flame are superposed in the resulting beam, and at a moderate distance the differences in the bands almost disappear. But with a compressed light, like that of the Electric lamp, there is not this sort of correction. One shaden band which at a few inches from the refractor was only 1/2 of an inch or less in breadth gradually opened out and was a dark band 6 inches wide on the screen at 46 feet distance; it had every appearance as if it would widen & hold its character more and more outwards, and I saw nothing that would indicate its disappearance at any distance. It would require the utmost perfection of workmanship to get rid of these bands.

The coloured bands are due to the difference in what is called the dispersion of the different rays of light. A single prism can & must produce them when the center of light is very small; and as the light is smaller & more powerful the colours are more intense. They were very bright upon the screen, and appeared as if they would travel out to a great distance still keeping their colour. With the oil flame of 1 1/2 inches high these coloured rays superpose & overlap each other; at the screen they are but small in effect & disappear at the distance of a hundred yards or so. Supposing the dioptric apparatus so perfectly worked & fitted as to produce no dark bands the coloured rays from the different prisms would overlap each other; but without the prisms were made achromatic (which they could not be because of the angles required) there is I think no probability of their disappearing when the E light is used[.]

So if the eye be carried from above downwards across the horizontal band of light & colours on the screen, it will pass from one colour to another; & from light to comparative darkness & then to light again. In like manner if a ship move from the horizon towards a light house with a similar lamp an observer on it will gradually see the light, sometimes of one colour & sometimes of another; but what is more important, he will see it lighter and darker; and I can easily conceive the possibility, that the mere lift of a large wave may raise the observer out of a strong ray into one of comparative weakness, or vice versa, & so give him the impression of an alternating or revolving light when it should appear fixed[.]

In order to correct this effect Dr. Watson proposed to place two lights, one above the other, that they might act the joint parts of the top & bottom of a lamp flame. This has not been tried. I conceive it would be very difficult to arrange them vertically one above the other; and if not vertical, it would require four or more lamps for one center, with separate batteries to each, & all the complications consequent thereon.

When the concentric lens was brought before the Electric light the effect was exceedingly beautiful and the illumination on the screen most vivid. The colours were less because here the rays from different parts of the lens were frequently overlaid & neutralized each other[.] Here again the influence of the central condition of the source was seen; for the illuminated space on the screen was not larger than the lens & it is probable that even at a great distance the divergence would be very little & the beam very narrow. The flame of the Fresnel lamp has a horizontal width of about 3 inches[.] The electric light is probably not not [sic] more than the 1/8 of an inch in width in the same direction so that the effectual divergence of a beam from the latter would probably be not more than a twentieth or a thirtieth part of one from the former. Hence, though very bright it would endure for so short a time in any one direction that its occurrence might be missed; what is of more importance is, that having a very small vertical width, accident or a want of correction in the height of the carbons might easily throw the beam altogether above or below the horizon, or the place of a ship on the sea.

Much, therefore, as I desire to see the Electric light made available in lighthouses, I cannot recommend its adoption under present circumstances. There is no human arrangement that requires more regularity and certainty of service than a lighthouse. It is trusted by the Mariner as if it were a law of nature; and as the Sun sets so he expects that, with the same certainty, the lights will appear. The means of meeting this expectation must be provided for the most isolated positions, where, if there be a derangement, there is no help to be had except that which is on the spot, and where, therefore, no chance of risk that can be avoided should be permitted to occur: and still further to explore such precautions it must be remembered that the most important lighthouses are often in the most out of the way localities[.] It would not be easy even to replace a failing Voltaic lamp by an ordinary lamp but would require much time. I think therefore the Electric light should be tried in its other applications that its conditions & liabilities may be eliminated, its construction perfected - and a school provided w[h]ere a body of men may be taught how to use it with certainty; before it is introduced into lighthouses whose present optical construction may not consist with it, and whose present attendants would as I believe be unable to fitly guard against or contend with its peculiar liabilities.

One other kind of doubt I am bound to place before the consideration of the Elder Brethren[.] The production of the light which the Electric company proposes to supply is based upon commercial considerations. A manufacturer requires nitrate of Iron which can be made at the manufactory as it is wanted and in quantity according to the demand. It is by the light scheme proposed to make it at 100 or more different & distant places, very often difficult of access, and in quantities not dependant upon demand in the manufactory but upon the light required in different seasons. For this the materials of which a large preparation and strong acids in glass are to be conveyed to these spots, men retained to use them there, & then the products removed and all for years together with the regularity of clockwork. How this is to be done remuneratively, except by an enormous charge for the light, I cannot imagine; and therefore, independently of all the precautions and contingencies which I have noted I should suppose that the Trinity house would not think of changing its system and the whole construction of its lighthouse illuminating arrangement or put itself in dependence upon a commercial company before it had had a few years assurance of the permanency and stability of the Manufacture on which it would be called upon to depend²[.]

M. Faraday

Royal Institution | 14 [sic] August 1854.