Faraday report to Trinity House   29 April 1859

Report on the Magneto-Electric Light established by Professor Holmes at the High Lighthouse, South Foreland.

Royal Institution, | 29th April, 1859.

The light applied in the South Foreland Upper Lighthouse is an electric light; not produced, however, by a voltaic battery, but by magneto-electric induction. In the year 1831* it was discovered that when a piece of soft iron, surrounded by a metallic wire, was passed by the poles of a magnet, an electric current was produced in the wire, which could be exalted so as to give a spark. The apparatus of Professor Holmes, which is figured and described in the accompanying paper A¹, consists of an accumulation of powerful magnets and iron cores with surrounding coils, accurately arranged, so that when the associated cores are revolving they send all their currents into one common channel, from whence they are conveyed to the lantern by conducting wires, and there produce the electric light. There is no consumption of material or energy, other than that of the burning fuel required at the steam engines to produce motion.

A trial of the light began in the lighthouse, on the 8th December, 1858; but as the apparatus was imperfect in some points and the results unsatisfactory, the lighting by the apparatus was suspended for a while, that the defects might, if possible, be remedied. The lighting was renewed on the 28th March instant, and has been continued regularly since.

I have had the honour of accompanying the deputy master² and brethren, both on the former and present occasion, so that I know personally what the light was and is. No report was made at the former time, because of the expected improvement of the arrangement; but it now becomes my duty to report on such matters regarding the lamp as properly fall within my province.

Being on board the yacht off Dungeness on the night of the 20th inst., about 21½ miles from the South Foreland, the weather being rainy and the sky covered with clouds, we could see (when the sun went down) the high light illuminated and appearing as a faint star. I could not perceive the low light; but proceeding eastward the low light gradually became visible, and the high light increased in brightness. When about S.W. by S. the yacht approached the lights more directly; then went eastward again, and after awhile turned and proceeded towards Dover; so that the observations were made through about one-fourth of the horizon, and at very different distances from the South Foreland. From the time the upper light was first seen until the last, it remained visible and steady (with the exceptions to be mentioned immediately), and much superior to the lower light when that also came in view. The exceptions were as follows:- At times the high light fell off, and once appeared almost out, when the lower light underwent no such change. This effect had been anticipated and was due to the following cause:- the upright bars of the lanthorn windows and the dioptric apparatus are much broader than the electric light, the latter being, indeed, not more than one-sixth of an inch in horizontal width; hence they throw deep and rather sharp shadows. By taking the bearings of these beforehand, it was found at sea, that the falling off of the light coincided with these shadows, and hence the above exceptions. The evil occurs in part with the central oil lamp, and is in some lighthouses partly remedied by causing the window bars and astragals to incline from the perpendicular, and therefore out of the plane of refraction. Other remedies (in addition to this) are applicable in the case of the electric light, and in the present instance the effect is lessened by the use of a small reflector at the lamp, close to and behind the light. The light was at all times white, or even blue occasionally, in comparison with the low light, which appeared yellow or reddish.

The next day I examined the lighthouse and apparatus both by day and night. The magneto-electric machines, steam engines, and steam-condenser were generally as at the last visit. In respect of the commutator it had worn very little; the application of a file to the surface of one wheel had removed about the thirteenth of an inch of metal since the apparatus was first erected, there remaining about 1½ inches still ready for consumption in like manner, if needed. During the day time I compared the intensity of the light with that of the sun; i.e. it was placed before and by the side of the sun, and both looked at through dark glasses; its light was as bright as that of the sun, but the sun was not at its brightest, and was only seen at intervals between clouds.

In the lanthorn, there was now but one electric lamp in place, two others, however, being on the wire rails ready for change of lamp at any moment. The magneto-electric machines were set in action, and the lamps were manipulated with, both by day and night, to my entire satisfaction. When the steam engines were ready for action the machines could be set in motion, and the current evolved within the space of half a minute; the lamp could then be lighted in an instant, and if it were required to put out and displace that lamp and replace it by another, the operation could easily be performed by one person within 10 or 15 seconds. The light may be considered as at its full intensity at once, though it was reported to me as growing up in power until about four o’clock in the morning, an effect probably due to the continued recurring inductive action in the cores and coils of the revolving apparatus.

The place of the light in respect of the dioptric apparatus is exceedingly well retained, even more so than in the case of a lamp flame, which, though its base be fixed, varies in its height. The light itself is not that flickering, wavering, revolving light produced by the voltaic battery, but, in a sheltered atmosphere, as in the lanthorn, is fixed in its position; a fact of great importance in the application of small catoptric or dioptric apparatus. An important regulator of the character of the light is given by two magneto-electric coils introduced into the circuit in the lanthorn. The light does not call for continued attention, but is often left untouched for one, two, or even three hours together. The eyes of the keepers are not affected, though the blue glasses provided for them are very pale in colour, for the light is better watched by observing the place and intensity of the rays which fall here and there on the walls of the lanthorn, than by looking at the light itself.

On going out to the hills round the lighthouse, the beauty of the light was wonderful. At a mile off, the apparent streams of light issuing from the lanthorn were twice as long as those from the lower lighthouse, and apparently three or four times as bright. The horizontal plane in which they chiefly took their way made all above or below it black. The tops of the hills, the churches and the houses illuminated by it, were striking in their effect upon the eye.

All the reports which have come in from the surrounding lighthouses, floating lights, and pilot vessels, confirm the superiority of the upper light above the lower, though many of the reports are imperfect. Those from Dungeness are the best; and include 160 observations made between March 28th and April 16th. The upper and the lower South Foreland and the Grisnez lights were either visible or invisible from the station, according to the weather. The upper South Foreland was visible first and oftenest i.e. always if the others were; then the Grisnez came oftenest in sight; and then the Lower South Foreland; but these were nearly equal. The Grisnez was sometimes in sight without the lower South Foreland, but never without the upper; and the lower South Foreland was sometimes in sight without the Grisnez. Upon the 160 observations there were 73 on which none of the three lights were visible; 86 upon which the upper light was visible; 40 upon which the lower light was visible, and 44 upon which the Grisnez Light was visible. When both the upper and lower lights were visible, the upper (except in four cases) is said to be twice the power of the lower; in the four cases it is called equal. There was one exception on April 3rd at 10 o’clock p.m., when the upper light is stated to have gone out. The report does not say whether it came in again, but the weather at 11 o’clock is returned as very thick and misty. Generally the light is said to be very steady.

The keepers at the Goodwin and South Sand head floating lights appear to have had a fair view of both upper and lower lights. If both were visible, the upper was much the most powerful; the upper was always visible. The lights were generally very steady; if wavering, both the upper and lower wavered at the same time, as if the cause were in the air, not in the lights. The reports from the Gull Stream Light are very poor and afford no instruction. There is no comparison or mention of the upper and lower lights.

In fulfilment of this part of my duty I beg to state that, in my opinion, Professor Holmes has practically established the fitness and sufficiency of the magneto-electric light for lighthouse purposes, so far as its nature and management are concerned. The light produced is powerful beyond any other that I have yet seen so applied, and in principle may be accumulated to any degree; its regularity in the lanthorn is great, its management easy; and its care there may be confided to attentive keepers of the ordinary degree of intellect and knowledge.

There are many considerations, beyond this establishment of the fitness of the light in principle and management for lighthouse purposes, regarding its introduction into lighthouses generally, on which I should hesitate to speak before those who are far more competent to judge of these matters than I am, were it not for the encouragement which the Brethren of the Trinity House give me, and especially as regards this light in respect of a letter from Professor Holmes, dated 28th April 1857, which I considered in my letter of the 1st of May³. I will therefore venture to enumerate some points which are against and others in favour of the light, and of a change in the present system. In the first place, the simplicity of the present system is very great compared with that of the electric light. Only two keepers are required to a lighthouse, they need possess no special knowledge, ordinary attention is all that is necessary; and thus failures of the light are almost impossible. In the new system a second set of men will be required to attend the engines; and there must be amongst them one or more who understand the principle and construction of the lamp in the lanthorn, of the magneto-electric machines, the steam engines, and the condensers below, and be able to make effectively the repairs necessary to the apparatus; or, as I think is more probable, a competent resident intelligent engineer, with his stock of tools and means, will be required.

In the next place, the expense of the new system must be large, compared to that of the present system. As outfit, there must be the cost of the two if not three magneto-electric machines, with the corresponding steam engines; of the houses to contain them; of the cisterns, condensers, and water apparatus, and supply associated with them; of the electric lamps in the lighthouses, with their connexions; and of the lodging houses for the extra men. And in respect of current expenditure, additional provision must be made for the wages of the extra men, with coals and such things as are allowed them; the fuel for the engines; the supply of water; and the repairs required by the wear and tear of all the apparatus. To these causes of expense must be added the claims of the Patentee. In the matter of expense for any given amount of light, I may say that the letters I have already referred to of the 28th April and 1st May 1857, can scarcely be accepted as giving sufficient information. I conclude that by this time Professor Holmes is in a position to give an amended statement.

Amongst partial objections it may be stated that the light cannot be expected to apply to all lighthouses, or receive general adoption. I am not aware how many it might be fit for; but there are evidently some at extreme situations where it would be objectionable in its present state. An objection has also been made, of which I cannot see the force; namely, that the light is too bright; that it gives a false impression of the distance of the lighthouse, and that it blinds the eyes of the mariners to the perception of the lights on board vessels between it and them. These objections, if they have any force, must be judged by mariners themselves.

The points in favour of the magneto-electric light, now that its practicability has been established, are strong and clear in relation to the increase of light and the advancement of lighthouses in power. In cases where the light is from lamp flames fed by oil, no increase of light at or near the focus or foci of the apparatus is possible, beyond a certain degree, because of the size of the flames; but in the electric lamp any amount of light may be accumulated at the focus and sent abroad, at, of course, an increased expense. In consequence of the evolution of the light in so limited a focal space, it may be directed seaward, diverging either more or less, or in a vertical or horizontal direction, at pleasure, with the utmost facility. The enormous shadow under the light produced by the oil flame burner, which absorbs and renders useless the descending rays to a very large extent, does not occur in the magneto-electric lamp; all the light proceeding in that direction is turned to account. The optical part of the arrangement, whether dioptric or reflecting, might be very small in comparison with those in use. It is probable that the system either of reflectors or refractors need not be above 18 inches in diameter, perhaps much less, and probably a foot high. The glass of the lanthorn through which the rays would pass would not then require to be above 12 inches radius, i.e. from the light. This would allow that part of the lantern to be constructed of carved plates of glass, framed above and below, and requiring no opaque, intervening, shadow-making uprights. There appears no reason why such a lanthorn should not enclose the electric lamp round seven-eighths of the horizon, and yet allow of all necessary attendance on, and change of the lamps. Such arrangements, however, could only be made when the lamp is (if ever) perfectly established; for they would preclude the substitution of an oil lamp for the electric lamp, if any accident occurred to the latter.

Before concluding this report, I must bear my testimony to the perfect openness, candour, and honour of Professor Holmes. He has answered every question; concealed no weak point; explained every applied principle; given every reason for a change either in this or that direction, during several periods of close questioning, in a manner that was very agreeable to one whose duty it was to search for real faults or possible objections, in respect both of the present time and the future⁴.

(signed) M. Faraday

* Philosophical Transactions 1832, p.131, Faraday⁵