Faraday report to Trinity House   21 February 1862

Royal Institution, 21st February 1862.

I had the honour of being with the Deputy Master¹ and Committee at the Dungeness lighthouse, on the 12th and 13th instant, for the purpose of examining the arrangements for, and the effect of, the Electric Light now established there, and it becomes my duty to make a report according to the result of my personal observation.

Communications of the dates, 29th April 1859², 29th [sic] February 1860³, and 19th January 1861⁴, regarding the Electric Light at the South Foreland, will sufficiently indicate the nature of the light, and the engines and machines required. At Dungeness, the Electric Light is now placed in an optic apparatus constructed especially for it, which is only 16 inches in height, and 14 inches in external diameter. The apparatus consists of six lenticular zones and seven reflecting zones; of the latter three are below and four above. At the South Foreland there was one electric lamp placed in the centre of a Fresnel optic apparatus. Here there are two of the new optic apparatus, placed one over the other in the axis of the lantern, and four electric lamps, for each apparatus two, only one lamp being used at a time. Mr. Holmes includes in his plan the use of all these lamps and apparatus, because of the facility of rapid change in the lamps, carbons, &c., and no doubt they are greatly advantageous in this respect. They cause no alteration in the magneto-electric machines, wires, engines, &c. which are the same as were employed at the South Foreland.

Beside the electric apparatus, the light from which passes through the upper panes of the lantern, the original reflectors and their lamps are retained in place, so that they can be at once substituted for the Electric Light, if any accident or failure should occur to the latter, and also may be used in conjunction with the Electric Light in a comparison of one with the other.

The four electric lamps appeared to me to be equally good and in equal acting order; they answered to the treatment applied to them; but must be considered as delicate apparatus, whether in regard to the attention necessary, or to their repair when out of order.

The two optic apparatus are so different from anything that has preceded them, that they cannot be compared with former apparatus. The glass was of good quality, and the workmanship excellent. Though they enclosed 275 degrees of the horizon, there was only one thin metallic joint in their length, and thus the cause of much interfering shadow was removed. The form and adjustment of the prisms seemed to be very good, i.e. the direction in which the light issued forth appeared to be quite right, so far as observation could be carried on, on shore and in the lantern. There was no good sea horizon visible by which the foci in the lamp centre could be tested; and besides that, a good experimental investigation on land will be required hereafter to determine the direction of the most intense ray of light horizontally, or upwards or downwards: or the parallelism or divergence or convergence, using the practical foci as now arranged. The lower surface of the upper carbon has been made focus, in relation to the sea horizon, for the refracting belt, and the upper reflectors; and a spot 1-8th of an inch above this lower surface made focus for the lower reflectors. Generally speaking, the most intense rays proceed downwards, the next intense, horizontally, and the weakest, upwards.

The prisms of the present optic apparatus do not intercept all the light that proceeds generally in their direction. In an apparatus so new and original as this is, I am prepared to accept them for the present case; but in future apparatus this escape should be provided against. The small metal reflectors on the land side, which are very valuable in their action, may be a little enlarged hereafter with advantage, so as to catch all the escape light passing in that direction.

I did not observe any damp on the glass of the lantern in the daytime (during my observations with the light), but at night-time it appeared. There is no source of water or damp in the light itself (as there is in oil or combustion lamps); on the other hand, there is no sensible source of heat to keep the lantern and window clear. The damp was on the windows, both inside and outside; and the stove was so arranged on the land side only, as to be of little or no use to the lantern generally.

There should be a screen over the lamps at the ceiling between them and the cowl passage outwards.

The lantern glass was not quite perpendicular; but that I consider as of no consequence, as the issuing ray goes straight on. The astragals interfered here and there; but if the system succeeds there will be no difficulty hereafter in constructing the lantern glass, so as to present no obstruction of this kind.

The steam engines, magneto-electric machines, water condensers, &c. appeared to be in order; and they were separated by six or seven doors from the lantern, and could cause no dimness there. The machine-room required the whitewashed ceiling to be covered up; the engines and steam-pipes required to be jacketted or felted. As suggested by the engineer in charge, there is abundant heat produced from the condensing steam of the engines, for any purpose required in the tower or lantern, which could be employed there by a proper arrangement of steam-pipes.

Arrangements were made on shore (Mr. Holmes being in charge of the light), by which observations could be made at sea about five miles off, on the relative light of the electric lamp and the metallic reflectors with their Argand oil lamps; for either could be shown alone, or both together. At the given distance the eye could not separate the two lights, but by the telescope they were distinguishable. The combined effect was a glorious light up to the five miles; then, if the Electric Light was extinguished, there was a great falling off in the effect; though after a few moments rest to the eye, it was seen that the oil lamps and reflectors were in their good and proper state. On the other hand, when the Electric Light was restored, the glory rose to its first high condition.

Then, whilst both were in action, the reflectors were shaded, and the Electric Light left alone; but the naked eye could see no sensible diminution; nor when the reflectors were returned into effectual use could it see any sensible addition to the whole light power; though the telescope showed that the alteration in the lantern had taken place at the right time. Such was the power of the Electric Light, that the addition or subtraction of the light of a fully effective set of reflectors, with their lamps, would not have been sensible to a mariner, however observant he might have been.

Then the reflector lamps were put out, and the electric lamp alone observed for the rest of the time. Went to the Varne Light, 12 miles off; as we receded, the Electric Light was fine, but after nine miles seemed to fall off. At the Varne, the fine revolving or flashing light at Grinez (12½ miles distant), when at its maximum, was equal, and, I think, superior to the Electric Light; but the constant light of the latter, and the intermitting light of the former, must be remembered. Returned in towards the Dungeness Light, and at about nine miles, and within, had again reason to think that that distance was more favoured (by the adjustments) than the sea horizon distance.

The colour of the Electric Light was very white, except when a little iron or antimony had collected as ash between the carbons, and for a few moments gave rise to a yellowish or pale reddish tint.

On the Thursday morning an examination took place, before the Deputy Master and Committee, of the lightkeepers, and their ability to take charge of the service of the lamps. They had been under instruction and in charge since the 31st January. It was soon evident that the lamp could not fairly be left to them. A certain amount of intelligence is required, and of willingness. Further, the present regulations as to authority, responsibility, &c., were adverse to a good result. I was very glad when the Deputy Master and Committee concluded that the care of the Electric Light ought to be made exceptional for the present; that two intelligent young men should be appointed keepers; that they, with the engineers, should be fully instructed; that after a certain time, when declared competent, they should be examined; and if found competent, the light should then be re-opened, with the engineer as chief keeper; the lighthouse service being in the meantime sustained by the use of the oil lamps and reflectors⁵.

(signed) M. Faraday