Faraday to George Biddell Airy   13 January 1844

Royal Institution | 13 Jany 1843 [sic]

My dear Sir

I ought to have written & thought about you long ago indeed the latter I have done only very imperfectly for I have not felt able to undertake thought, but I will not pursue the bad employment of excusing any longer for there is no profit in it, so will turn to the matter of registering¹. I do not see the way to a perfect register & the different conditions of the atmosphere in relation to <e>lectricity but fancy one must work the way out by degrees and at present the point & revolving cylinder is the mode of registering I should first work at[.]

Now look at the pen & ink sketch accompanying this & say what you think of the general notion. a a light circular disc of wood covered with tin foil or other conducting matter fixed on an arm c & balanced on the axis e by the counterpoise d & float l the axis or edge e is supported on a foot of shell lac f to effect insulation g.h.e rests on the arm c & moves with it g is shell lac to insulate i is the registering point - k is a silk thread covered with shell lac if needful supporting the float l (of metal, glass or other convenient substance) & this float is partly immersed in a fluid m contained in a jar n & that is inclosed in a box t - r is a De Luc² or Zamboni's³ pile⁴ the lower end uninsulated the upper connected by a bender s to the pivot on which a c & d move so that they are always electrified Pos. - b is a disc similar to a but fixed & connected by the wire o with your insulated air rod so as to take on the same state as it - p is a box enclosing the principle parts of the apparatus the box being of wood well varnished inside so as to give no moisture to the space within & q is a basin containing chloride of calcium to keep the air perfectly dry - the good & enduring effects of this I know. Under these circumstances a will always be electrified & I hope regularly & b electrified by the air will generally attract it, because of its opposite state & more or less according as it is more or less electrified & this may be registered by the motion of i - the float l regulates the motion on each side of the normal position. Now there are many quantities in this apparatus varying with the relative charge & position of the two plates a & b but it seems probable that all these may be experimentally compensated for - thus supposing the charges in a & b constant when the attraction has brought them nearer together more of the weight of the float b is sustained by a than before and thus at the same time that it keeps a & b apart gives the varying positions of i which are to record the attractions. But as a & b approach each other their actual charge will vary for a will become more positive towards b & b more neg towards a than before[.] Hence the attractions will be stronger than they ought to be[.] This I thought might be compensated for probably by giving A a curved form deduced either mathematically or by experiment.

Again if b were continually insulated its state at a given moment would not necessarily measure the state of the air at that moment for it might be the state given by the preceding periods of time - but could not this be obviated thus[.] Suppose a watch or clock movement which should make a wire touch o once in a minute or half minute to uninsulate it & then immediately leave it insulated again: in the interval the point i would measure the electric state of the air for that interval - none of the effect of the preceding interval remaining[.]

Perhaps it might be better for the disc a to have a perfectly insulating arm c so that its electricity could not travel to & fro to d & back by the effect of induction from b . Then the same clock movement might at the same instant uninsulate b and connect r with a to charge the latter with a given amount the next moment the movement would separate r from a & leave b insulated b would receive its charge from the air & find in a a constant quantity of electricity & by attracting or repelling it measure its own force. Under

such circumstances the state of a would much better fit it for a measure of the electric force because the electricity could not then leave that disc[.]

Of course adjustments would be required to regulate the height of the jar n & its fluid - the intersection of the fluid & float the bearing of g & other points. I imagine there would be no want of power to move the index or pencil i because the discs a might be a foot or two in diameter & if one instrument was too delicate to measure the stronger charges from the air, two or three different degrees of range might easily be connected so that the action should throw each other into use in their proper order.

And now I will stop for if the plan be really practical the best thing would be to have something made & then observe experiment correct & improve.

Supposing the float bad one could easily use springs & put the discs in a vertical position or change about twenty ways. My object has been by the attraction of two discs (& repulsion) to be able to move a point & register as you do wind on a cylinder[.]

Ever My dear Sir | Yours faithfully | M. Faraday

G.B. Airy Esq | &c &c &c

Address: Geo Biddle Airy Esq | Astronomer Royal | &c &c &c | Observatory | Greenwich Park