Faraday to George Biddell Airy   9 March 1844

Royal Institution | 9 Mar 1844

My dear Sir

I have [been] working to procure for you a few practical results a<s> to the force of attraction between planes one being electrified¹

a is a board covered with tin foil - square & 24 inches by 36 - <&> is fixed - insulated - connected with an electrical machine and also with a ball of brass which is opposite to another ball the latter being uninsulated when the machine is moved it charges a until the intensity is such that the spark passes at the balls. This distance was alterable & I will call it s. b is a light disc made by stretching cartridge paper over a childs hoop & covering the paper with tin foil it warped a little but a tension wire across the upper edge of the hoop easily adjusted it into a flat plane. This disc was suspended from a balance uninsulated & adjustable as to distance from a. I will call this distance t. Making s a constant quantity & putting a certain weight into the balance I electrified a & altered the distance of the disc until the utmost charge a could receive was just enough to pull the disc b down - the following are the attractive forces in grains

Surely this is power enough for the registering apparatus which I proposed² especially as we might easily have the attracting surfaces five times as large or even larger if needful. I have made the fixed surface electrical but either might be so. In practise it would probably be best to place them vertical & box them up.

Ever dear Sir | Very Truly Yours | M. Faraday

The Astronomer Royal | &c &c &c

Disc 2 feet in diameter - attractive force expressed for given distances in eights of an inch by number of grains in weight.

Charge in the sustaining Jar - or in cloud or in paint

*Spark across 1/8 of inch between balls 3/4 inch in diameter in air

When the distance remains the same the attractive force is as the square of the electric charge in the sustaining Leyden Jar[.]

When the charge in the sustaining jar is constant & the distance varied the attractive force is as the squares of the distances inversely[.]