William George Armstrong to Faraday   25 October 1840

Newcastle upon Tyne 25 Oct 1840

Dear Sir,

The day after I last wrote to you¹, I examined two high pressure boilers at Kellingworth Colliery, I found the steam which issued from the safety valves of both of them, sufficiently charged with electricity as produce faint sparks when treated in the manner I adopted in the other cases. These boilers like those at Seghill were both supplied with colliery water, & the valves were loaded with about 40lbs on the square inch. Although the sediment from this water was great, it was not equal to that deposited by the Seghill water. On the following day I tried the steam which was discharged from the valve of a locomotive boiler, loaded with about 50lbs on the sq inch, & this I found to be more highly charged with electricity than any I had seen, except that at Seghill. The water used in this boiler was from the River Tyne, the water of which is tolerably soft, & produces no incrustation in the boiler, but deposits, as I am informed, a soft dark colored substance. I next tried a common high pressure boiler, which was supplied with a mixture of rain & spring water, & the valve of which was loaded with 30lbs on the inch, & here also I obtained electrical sparks, though not nearly such strong ones, as those afforded by the steam of the locomotive. I am quite at a lost what inference to draw from these discordant results. The steam evolved from the Seghill boilers, in which water is used strongly impregnated with mineral matter, is pre-eminently charged with electricity, while the steam from a neighbouring boiler in which rain water is used shows no indication of electricity. This fact seems to point to the mineral nature of the water as the cause of the phenomenon, but on the other hand at Kellingworth, where the water used for the boilers is also of a strong mineral character, the electricity of the steam is very feeble, & in the case of the locomotive boiler which is supplied with water of quite an opposite description, electricity is very copiously developed. Neither do the different quantities of electricity obtained in the several cases, appear to bear any relation to the pressures, for at Seghill where the pressure is only 35lbs the electricity is most abundant; whereas in the rain water boiler, where the pressure is 40lbs, no electricity could be found.

I have little doubt you will meet with similar phenomena, in a greater or less degree, if you try some of the locomotive boilers, at any of the London railway stations.

Unless the electricity be developed in the steam in the act of issuing from the boiler, it appears to me that by inserting a glass tube in the top of the boiler, & causing it to protrude a short way into the steam chamber, a jet of steam would be procured much more highly charged with electricity than any that can be obtained from a simple aperture in the boiler; because by this means we should avoid drawing the steam from the superficial stratum which lies contiguous to the metal, & which must thereby be deprived of much of its electricity; & we should also prevent the abstraction of electricity from the steam which must take place in its passage through the metallic orifice. With this view as well as for the purpose of collecting the electricity of the steam more effectually than can be done by the methods I have hitherto employed, I am getting an instrument constructed, of which I subjoin a sketch, & of which the following is a description.

A is a cock which is to be screwed into a hole to be drilled in the top of the boiler, & the passage through which cock is sufficiently large to admit the glass tube B into the boiler, when the key is turned into the proper position. The upper end of the cock is furnished with a stuffing box to prevent any escape at that point. Upon the top of the tube B is affixed a vessel consisting of two thin copper cylinders placed one within another, a small interval being left between them for the passage of the steam. A stop cock is placed at the junction of this vessel with the glass tube, & a small pipe, C, conveys the steam from the stop cock into the upper part of the inner cylinder. The course of the steam from that point is indicated by the arrows, & it will readily be seen that the steam, by passing in a thin film between the inner & outer cylinder will communicate its electricity to this vessel, which being insulated by the glass tube, will answer all the purposes of a prime conductor. This will certainly be an electrifying machine of a very novel & economical construction.

From the reiterated accounts I receive, I am quite satisfied I have not yet seen the Seghill Boiler in any thing like the highly charged state in which it was, shortly before I first saw it. Hoping to hear from you shortly on this most interesting subject[.]

I am Dear Sir | Very respectfully yours | W.G. Armstrong

M. Faraday Esq

The use of the cock A is to afford a ready means of stopping the opening when the tube is withdrawn, & also to facilitate the insertion of the tube, which could not otherwise be passed into the boiler except when the water was cold.