Faraday to Jacob Herbert   24 January 1853

Royal Institution, | 24th January, 1853.

My Dear Sir,

In reference to the remarkable stroke of lightning which occurred at the Eddystone Lighthouse, at midday on 11th January of this year, and made itself manifest by a partial flash discharge in the living rooms, I have to call your attention to the drawing herewith returned, and to the circumstances which appear (from it) to have accompanied and conduced to the discharge¹.

In the body of the stone work above the store-room exist eight rings of metal; each going round the building, and each being four inches square of solid iron and lead. Also, latterly the bed-room and sitting-room have been lined with a framework of iron bars, situated vertically, and pinned by long bolts into the stonework.

The part of the tower above the floor of the living-room is, therefore, filled with a metallic system, which, with the metal lantern, gives a very marked character to the upper half of the structure.

The recent metallic arrangements (but not the rings) are connected with the lightning rod; and the copper part of this rod, beginning at the floor of the living-room, then proceeds downwards by the course which can be followed in the drawing, and terminates on the outside of the rock between high and low water marks.

Considering all these circumstances, I was led to conclude that the conductor was in a very imperfect condition at the time of low water; and I had little doubt that I should find that the discharge had taken place when it was in this state, and very probably with a spring tide.

The day of the stroke was the 11th January - a new moon occurred on the 9th, so that it was at a time of spring tide.

The occurrence took place at midday; and, according to the tide tables, that was close upon the time of low water at Devonport. The end of the conductor would then be 6 feet from the water, if the latter were quiescent, and I cannot doubt that this circumstance gave rise to that diverted discharge which became so manifest to the keepers. Mr. Burges², with whom I have conversed about the matter, thinks it probable that, through the violence of the waves, the conductor does not now descend so much as is represented in the drawing.

I think it essential that the lower end of the conductor be made more perfect in its action; and I should prefer this being done on the outside of the tower and rock, if the rod can be rendered permanent in such a situation.

If it be impossible to prolong and fix the lower end of the conductor where it now is, so that it shall have large contact with the sea at low water, then I would suggest, whether or no, on the more sloping part of the rock, about midway between high and low water, three or four holes could not be sunk to the depth of 3 feet, and about 3 or 4 feet apart, and that copper rods being placed in these, they should be connected together, and the lightning rod continued to them.

If this cannot be done, then it might be right to consider the propriety of the making a hole through the centre of the building and rock, about 2 or more inches in diameter, and 30 feet deep, and continuing the conductor to the bottom.

A conversation with Mr. Burges regarding the present state of the Bishop’s Rock Lighthouse, now in the course of construction, induces me also to suggest the propriety of making provision for the lightning conductor as the work proceeds.

It would be easy now to fix terminal rods of copper, and to combine them upwards with the work. Considering the isolated and peculiarly exposed condition of a lighthouse on this site, I would propose that there be two conducting rods from the lantern, down the outside on opposite sides of the tower, each terminating below in two or three prolongations, entering as proposed into the rock, or into fissures below low water mark, so as to be well and permanently fixed³.

I am, &c. | (signed) M. Faraday

The Secretary, | Trinity House.