William Thomson to Faraday   20 November 1860

2 College, Glasgow | Nov 20, 1860

My dear Faraday

Mr White¹ who has made my electrical instruments is ready to make them to order for any one². He is now engaged in the construction of a complete set for atmospheric observation ordered by Professor Joseph Henry of Washington, at a cost of £45. This includes <³> a divided ring reflecting electrometer, and <⁴> a guage [sic] electrometer to measure the electrification of its index; along with a waterdropping collector like the large one I had at the Royal Institution⁵ which was made at Kew; also <⁶> a portable electrometer.

The portable electrometer is not essential for the self recording apparatus which requires only electrometers <⁷> & <⁸>. The cost of the portable electrometer is £12 at Mr. White’s present rate; and that of the complete self recording apparatus £33. (not including however as you will perceive, clockwork and photographic apparatus.)

For ordinary use in a private house or a meteorological station where self recording apparatus is not required, electrometer <⁹> costing £13, and a small water collector which could not cost more than £1 or £2 is very convenient and accurate. This electrometer I have also found very useful and convenient in a great variety of laboratory investigations. I have it now daily in the hands of students measuring the intensities required to produce sparks (which, as you long ago found them, manifest very large, & not easily explained irregularities) finding the capacities of Leyden phials, measuring the intensities of galvanic arrangements, &c. For lecture room illustration however, & for laboratory work requiring a very sensitive electrometer, the divided ring reflecting electrometer is best.

I have often urged Mr. White to try to make the electrometer at a lower cost: but hitherto their construction has been very expensive. He promises me a new estimate in a few days when he sees his way through his present order; & I hope it may be somewhat lower. I shall send it to you when I get it.

As to the photographic curve, I have no explanation farther than that the marking of the neutral line was not yet arranged for. Whether it would have been on or off the paper I cannot say: probably about the edge of the paper farthest from the curve I should think. As it was probably when in fair weather, the curve must indicate varying positive & therefore the tails you refer to on the right of the track must have indicated quick augmentations above the average and returns to it, many times in the course of a period of 5 or 6 minutes. As the rate was only one inch per hour; the photograph does not prove these to have been abrupt as indicative of discharges. They may have been so, but they may perhaps more probably be the results of rapid not abrupt changes such as I have often observed in fair weather & which I think must be owing to gusts of electrified air about the building, or undulations in electric strata of air at no great distance overhead. It is curious to observe that these tails to the right are only during the 3d 4th & 5th hours. The track during the rest of the period presents zig zags on the whole very symmetrical as to right and left.

I hope soon to have some light about the electromotive properties of zinc and copper but at present it is a most perplexing subject.

Believe me Yours always most truly | William Thomson