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The following extract of a letter received from Professor 
VV. Thomson, F.R.S., Honorary Member of the Society, 
&c., was read by Dr. Joule. 
“ I have a very simple ‘ domestic’ apparatus by which I 
can observe atmospheric electricity in an easy way. It 
consists merely of an insulated can of water to set on a 
table or window sill inside, and discharge by a small pipe 
through a fine nozzle two or three feet from the wall. 
With only about ten inches head of water and a discharge 
so slow as to give no trouble in replenishing the can with 
water, the atmospheric effect is collected so quickly that 
any difference of potentials between the insulated conductor 
and the air at the place where the stream from the nozzle 
breaks into drops is done away with in my apparatus at 
the rate of five per cent, per half second, or even faster. 
Hence a very moderate degree of insulation is sensibly as 
good as perfect, so far as observing the atmospheric effect 
is concerned. It is easy, by my plan of drying the at- 
mosphere round the insulating stems by means of pumice 
stone moistened with sulphuric acid, to insure a degree of 
insulation in all weathers, by which not more than five per 
cent, per minute will be lost by it from the atmospheric ap- 
paratus at any time. A little attention to keep the outer- 
part of the conductor clear of spider lines is necessary. The 
apparatus I employed at Invercloy stood on a table beside a 
window on the second floor, which was kept open about an 
inch to let the discharging tube project out without coming 
in contact with the frame. The nozzle was only about two 
feet and a half from the wall, and nearly on a level with the 
window sill. The divided ring electrometer stood on the 
table beside it, and acted in a very satisfactory way (as I had 
supplied it with a Leyden phial consisting of a common thin 
white glass shade, which insulated remarkably well, instead 
of the German glass jar — the second of the kind which I had 
tried, and which would not hold its charge for half a day). 
