Faraday to George Herbert   13 September 1863

Royal Institution, 13th Sept, 1863.

Sir,

I received your letter of the 3rd Instant, accompanying a communication in M.S. from the Board of Trade, requesting me to consider the communication and<r> return it, with any observations I might have to make upon it’s contents. This I now do.

The communication is a translation into English, of an account of the Magneto-Electric Machine, the property of the Alliance Company (in Paris).

The account is drawn up in the manner of a professional opinion by the Abbe Moigno¹. I have numbered its pages that I might refer, when needful, to it. It contains errors, as you will see, at page <(7)>. It refers to many purposes and uses, to which the machine may be applied (21-24), but I have only to consider it’s application in a lighthouse. It speaks of experiments and developements [sic] still going on <(30)>, in such a manner, as to show that it’s practical use as a source of light, is not as yet fully wrought out <(9)>. I saw this machine at the International Exhibition; and then wrote to the Trinity House a letter dated 23rd June 1862²; in relation to a communication addressed to the Corporation from M. Berlioz³; which letter I will refer to on this occasion.

Though the principle of the French machine and Holmes machine is the same, yet there are special, practical differences; The French machine is much smaller and lighter. The induction bobbins are the part[s] which rotate in the French machine; they are much lighter than the permanent magnets, and constitute the inner part of the Apparatus. In Holmes machine, it is the heavier, outer magnets which rotate; so that the moving part is much lighter and smaller in the French machine, than in the other, and is more central and enclosed. The French machine has no Commutator <(11)>; so that the currents which go to the lamp do not merely intermit and recur, but also alternate in direction. It is concluded (in the communication) that not only is the machine better without the commutator; but also, that the problem of lighting was not solved whilst it remained <(16)>.

The alternation in direction of the Electric currents, causes a change in the circumstances of the lamp. The rate at which the upper and lower carbons will be consumed will be nearly alike. The direction of the maximum rays of light will now, not be downwards only, but both upwards and downwards, or perhaps nearly horizontal; but this will make no difference in the construction or efficacy of the optic part of the apparatus.

That the Magneto-Electric Light is admirably fitted for application in lighthouses, is not only a conclusion to be drawn from principle but one that has been thoroughly established by practice, at the South Foreland and Dungeness Lighthouses; i.e. so long as the knowledge, attentions, and precautions, are secured that are necessary for the uninterrupted command and security of the light. That being admitted there are then three points, in respect of which the two machines may be compared.

In the first place the kind of knowledge and attention required, to render the Magneto-Electric light constant and sure, is far above that necessary for lighting operations with the most elaborate and perfect oil lamp. These are now in course of practical trial at Dungeness. I believe that the Trinity House (with which the judgment in this respect, very properly rests) are not fully satisfied, that they are sufficient, or can be made so. There is nothing in the French apparatus which diminishes the necessity for these precautions, or gives any advantage to the one system over the other. Until this has been decided by experience and practice the second and third points hardly come into view.

The second point is the efficacy of the machine; This can only be learnt by long and continuous trial;- and, when for lighthouse purposes, in the lighthouse itself. It is not trial for a few hours, or a day, or a week, or even a year, that can settle this point between the two machines. The paper from the Alliance Company points to the many applications of the Magneto-Electrical Machine which are contemplated and considered practical (21-24). Many of these would give results of great consequence to the consideration of the application to lighthouses; for interruptions, if they occurred, would be of little consequence by comparison, and help, if needed, might be close at hand. Yet such processes are not established <(25)> and so there is no evidence that can be quoted. A lighthouse from it’s special nature;- it’s great importance;- it’s uninterrupted action; and its isolated position, seems to me to be the last place, to which processes comparatively new in their nature, should be applied; if there be any other educational position which can precede such application. One would naturally look to France itself for some real evidence of this kind in relation to the French machines.

The third point is cost. Certain results as to the French Apparatus are given (17, 18, 28, 33,) but the whole cost (outlay and current) can only be learned by experience in a lighthouse⁴.

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

George Herbert Esq | &c &c &c