Faraday report to Trinity House   1 May 1857

1st May 1857.

[1. I [Holmes] will endeavour to answer your letter of 22d instant in the best manner I can, hoping at the same time that the Elder Brethren will be satisfied with my answers as far as they go. As, without seeing the particular locality, it would be impossible, perhaps, to say in that particular case how I would adapt the machine. But in hardly any possible case can there arise any real difficulty in adapting the machine, as the light may (where necessary) be at a considerable distance from the machine without much lessening the amount of light.]¹

1. In order to present such observations upon Professor Holmes’ letter, as arise in my mind, I have numbered the paragraphs of his communication, and hope that a reference to them by figures will be clear and sufficient.

[2. The cost of a single machine and steam engine would be about 500l. The steam engine supplied with the machine would be of the most simple construction, acting directly on a crank fixed to the axle of the machine. No strap or band would be employed.]

2. The cost and arrangement of the machine can only be tested by practice.

[3. The expense of the light would be about from 1½d. to 3d. per hour, for coal and carbons, and the pay of a skilled workman until the ordinary lightkeepers would be sufficiently instructed by him to maintain the light without his assistance.]

3. The expense of the light can only be learned by a continued practical trial.

[4. If the Elder Brethren will allow us to place an Electric Light in any lighthouse where there would not be any great expense incurred for building, &c. for (say) three months, we will be at all expenses (and the ordinary lights need not be disturbed during that time) on condition that at the expiration of the three months the Corporation will adopt the light generally in their lighthouses if it fulfils the following conditions to their entire satisfaction, viz.]

4. It would not be safe to give any pledge of general adoption; the application might be very difficult and uncertain in the results at out-of-the-way stations. Better far to pay a sum for expenses of trial, and remain at perfect liberty to reject or apply the lamp according to circumstances. The Trinity House only can be the safe, as they are the responsible, judges for any and every case.

[5. First, that it costs less than the ordinary lights.]

5. Does this statement mean that for the same amount of light it will cost less? or that the lights, referred to in pars. 3 and 8, as the Electric Lights, cost less than the first class three-wicked lamp referred to in par. 8, or does it mean that the sum of all expenses, including interest of capital invested, wages of intelligent workmen, fuel, &c., will be less for the electric lamp of fivefold light power (8 & 9), or less for an equal proportion of light power?

[6. Second. That it is more effective in bad or foggy weather.]

6. No doubt true for the fivefold light power.

[7. Thirdly. That during that time it is not stopped on account of accident for any time equal to that taken to trim an ordinary lamp.]

7. In relation to this condition, will it not be necessary to have a second magneto-electric engine ready to supply the light, when, in course of time or by accident, the one in use might be thrown out of employ? Would not a second steam-engine be required for the same reason? Manufacturers are accustomed to keep an extra boiler ready for work, with steam engines, on such occasions.

[8. I will here, sir, beg you to lay before the Elder Brethren the results of certain photometric experiments made with a view to ascertain the comparative powers of a first-class three-concentric wick lamp and our Electric Light, as it will serve as a demonstration of the assertion made above.]

[9. The first experiment was for the comparison of the quantity of light given off by each, and was made at 12 feet distance. The results were-

Quantity of Light.

Oil lamp 1

Electric ditto 5½

As the distance was increased so did the difference increase also; and thus I was enabled to arrive at the intensity.

Intensity of Light.

Oil lamp 1

Electric ditto 5,625 ]

8 and 9. Can say nothing about the estimate; it is an experimental result. But the light I saw was considerably greater than that of the French lamp.

[10. By intensity is to be understood that property of light which enables it to penetrate to a distance; and, taking the above example, suppose that the two unassisted lights are viewed from a distance, on a foggy night, and that it is found that the oil lamp could usefully penetrate that fog to the distance of 100 yards, then the Electric Light would be equally visible at the distance of 100 x √5,625 = 7500 yards.]

10. Needs a practical result for its illustration.

[11. No quantity can ever make up for the want of intensity. For were there a hundred of such oil lamps employed, where one would become invisible, from distance or fog, each one would equally become so.

Intensity, therefore, and not quantity, is that property required in a lighthouse, and this property exists only in the Electric Light to any great degree.]

11. Not altogether so. If it were, then a common Argand lamp would be visible as far off as a concentric wick lamp; and six lamps and reflectors in one cant of a revolving apparatus would not penetrate, to a seaman’s observation, further than one lamp and reflector. Without reasoning on intensity and quantity, what is wanted is, simply, that light which is either more luminous or more visible at greater distances than another, to the eye of an ordinary observer.

[Holmes: 12. When the Electric Light is employed with a reflector, the quantity of light thrown off from it is much greater than when an oil lamp is used. Because, as the reflector only repeats the light of any lamp in proportion of - area of flame, divided into area of reflector - and as our area of flame is more than a thousand times less than the area of the oil lamp, the light given off would be a thousand times greater, even though the original unassisted lights were equal in quantity.

(signed) F.H. Holmes.]

12. Not so. The quantity of light reflected is not in greater proportion. The good effect (which is undoubted) depends upon the power of diminishing the divergence, and throwing almost all the light into one intense central beam, instead of spreading it into a wider, weaker beam.

I hope a situation may be selected where the magneto-electric lamp can be safely and effectually tried, for a time and under circumstances during which all the liabilities may be thoroughly eliminated. The light is so intense, so abundant, so concentrated and focal, so free from under shadow (caused in the common lamp by the burner), so free from flickering, that one cannot but desire that it should succeed. But it would require very careful and progressive introduction; men with peculiar knowledge and skill to attend it; and the means of instantly substituting one lamp for another in case of accident. The common lamp is so simple, both in principle and practice, that its liability to failure is very small. There is no doubt that the magneto-electric lamp involves a great many circumstances tending to make its application more refined and delicate, but I would fain hope that none of them will prove a barrier to its introduction. Nevertheless, it must pass into practice only through the ordeal of a full, searching, and prolonged trial².

(signed) M. Faraday