Faraday report to Trinity House   3 April 1857

Royal Institution | April 3 1857

Sir

Though I cannot pretend to say any thing that was not evident to the Deputy Master¹ & Elder Brethren as well as myself, on the night of Tuesday last², when certain lights at Blackwall Wharf were observed from Woolwich, still it may be considered as part of my duty to offer the following observations.

The first trial was made with the two pannels (containing four lenses) of the Bishops light apparatus, which had been before examined at the Trinity house (see letter of the 19 Decr. 1856³); the object being to ascertain that the expected alternation of illumination & darkness was certain. The results confirmed that expectation & the former conclusion. Each illumination had but a moderate degree of intensity (not more than half that of the great lens was to be expected); for only a twenty fourth part of the light of the whole circle could be received on each lens; i.e. the 1/24th part of the light from the central four-wicked lamp.

Then two successive casts or faces of the Buchaness light were exhibited. Each face consisted of three Argand lamps in Parabolic reflectors; the arrangement being such that eight faces occupied the circle.- The illuminated & dark intervals were here very good & distinct, as was to be expected. The light from one face of three lamps was much better than from one lens of the Bishops light apparatus; but that was to be expected also; for now 24 lamps fill the circle & an eighth of that (or 3 lamps) sent all their rays to the eye at once; whereas when the refractor was employed, a lamp equal to only 12 or 14 Argand lamps was in the center and only 1/24th part of its light was sent to the eye in each flash.

The Bishops light pannels were then again employed, but the central lamp was changed for the electric light of Professor Holmes: An extraordinary alteration in the results was now evident. As the apparatus revolved, a beam of light issuing from it was seen to sweep round, which when it came up to the eye, struck it for an instant with a beautiful & intense light and then passed away on the other side. The brilliancy at the moment was far above that of either of the former arrangements. This was chiefly due to three circumstances; first the greater amount of light from the electric lamp than from the concentric wick lamp;- next the concentration of the light into a small space, not more than the third or fourth of an inch in diameter; and lastly the absence of any thing like the great burner, which causes so much shadow in respect of the lower rays of light from the four wicked lamp. It is the second circumstance or smallness in size of the light itself, which allows every part to be brought nearly into the focus of the lenses, causes the brilliant ray already described to be sent forward to the observer at a distance. The divergence of the issuing beam is greatly diminished and its intensity proportionately increased.- The duration of the light is of course diminished also. The power of gathering up the light into an intense brief beam, which is obtained by the electric lamp, is unattainable, as I believe, by any lamp dependant⁴ on the combustion of oil or fuel of any kind.

The electricity for this light was procured, not from a chemically acting voltaic battery, but from the power of moving magnets, in what is known as a magneto-electric machine; a two-horse steam engine is required to move it, but there is no consumption of the parts, otherwise than by ordinary wear and tear. When the lenses were removed and the electric and concentric-wicked lamps placed in sight at once, the former was evidently much the brightest, but Dr. Holmes has not as yet made any comparison of the two, or given expressions of their relative illuminating power. Being examined near at hand, I found the Electric Light very pure and white, and very steady; there was none of the variation or flickering which occurs with the voltaic light; what alternations existed were so small, constant, and numerous, that the result to the eye was a perfectly steady illumination⁵.

(signed) M. Faraday