Faraday to James Cosmo Melvill   5 September 1839¹

Royal Institution | 5th September 1839

Sir,

I have the honor to acknowledge your letter and the papers and having read the latter beg leave to state that my opinion is in favour of lightning conductors². It is no doubt true that low rounded buildings such as I understand the powder magazines in the East Indies to be [are] but very little liable to be struck by Lightning but then if they are struck the destruction and injury may be very great. It is also I think very probable that a lightning conductor may under certain circumstances cause an electric discharge to take place where none would have occurred no conductor being present though on the other hand there is some evidence to shew that conductors cause a diminution in the number of electric discharges to the earth at a given place. It is also very certain that a badly erected conductor is worse than none and may cause great injury. But notwithstanding these considerations I have the strongest conviction in my mind that conductors well applied are perfect defenders of buildings from harm by lightning[.] Dr. O'Shaughnessy's³ papers are very valuable and serve to confirm my previous impressions, but it would be impossible for me to go over the whole of the opinions and evidence sent me without at the same time going into a far greater mass dispersed here and there. I would rather refer you at once to M. Arago's popular view of the subject in the Annuaire for 1838 p.p. 221. 549⁴ &c with which I in almost every point agree.

I would certainly recommend copper conductors instead of iron for the former metal conducts electricity almost seven times better than the latter. When struck it not only conducts the shock much better but in the predetermination of the stroke it determines more of the electricity to itself than otherwise would fall upon it and therefore tends in any case of a divided shock to leave less to fall elsewhere in its neighbourhood[.]

I should prefer them pointed. I should not put them far from the building at their upper extremity or in their courses downwards but the part that is underground I should turn from the building in its course through the earth and take especial care by plates of copper to make its contact with the moist earth extensive and good[.]

Conductors should be of certain height in relation to the roof or summit of the building to be defended. A lightning rod rising ten feet above any part of the roof or chimneys of a house might defend that house perfectly if close to it but not if ten feet from it. A rod rising fifteen feet above the highest parts of the roof would be more sure than one of ten feet. A rod projecting ten feet which would protect a building of a certain horizontal extent might not protect a building ten feet wider and a lightning rod has been considered as able to protect objects perfectly when they are not more than twice the distance from it of its height above them but for this to hold true these objects should not be themselves parts of large masses of metal approaching by their position and connection to the character of bad lightning conductors.

I have no fear of lateral discharge from a well managed conductor. As far as I understand lateral discharge it is always a discharge from the conductor itself; - it might be very serious from a badly managed conductor (and in fact makes them worse than nothing) but with a good lightning rod it can be but small and then not to badly conducting matter as wood or stone but only to neighbouring masses of good-conducting matter as the metals which either ought not to be there or if they are necessarily present ought to be in metallic communication with the lightning conductor itself. I am not aware that lateral discharge can take place within a building when a lightning conductor outside is struck except there be portions of metal as bolt wires or bolts &c which may form an interrupted conducting train from the conductor to the interior. It is true that cases which came under the denomination of returning stroke might perhaps produce a spark in the interior of a building but the phenomena of a returning stroke cannot occur at the place where the lightning strikes a conductor.

In my opinion a good conductor well connected with the earth cannot do harm to a building under its protection i.e though it may induce a discharge upon itself it cannot induce a discharge on the building: and the discharge on itself cannot give rise to any secondary effects which are likely to place the building in more danger than it would have been subject to had the conductor not been there[.]

I am Sir | Your obedient Servant | Signed | W. [sic] Faraday

Jas. C. Melvill Esq | &c &c &c