Peter Barlow to Faraday   6 April 1822

Royal Military Academy | Ap 6th 1822

Dear Sir

I am much disappointed to find I shall not have the pleasure of your company to day, I wished much to show you my experiments and results on the laws of Electro magnetism[.] However if S H D will have the goodness to show you my paper¹ you will I think easily comprehend what I have been about, and if while you have it, you would ask S H D permission to transmit it to me by post for an hour or two, there are one or two things which I wish to speak of rather more particularly than I have done. This is in reference to the 12 cases I have mentioned as resulting from the formula

²

one or two of which (by the bye) are at variance with what you state in the fourth paragraph of your first article in No 23³ - where you say “but if the wire be continually made to approach”⁴ &c the effect diminishes & it becomes small & the needle is indifferent to the wire[.]

According to my results both theoretical and experimental, this can only happen on both sides of the wire when the deviations at the north or south is less that 45° - if it be equal to 45 or greater; then what you state happens only on one side of the wire; but on the other, the effect continually increases and the needle will ultimately be reversed. It is one or two these beautiful coincidences between the theory and experiment that I wished particularly to show you and which I wish to put in rather a strange point of view than I have done in my paper, if I could be favoured with it again for a few hours. I hope you or Sir H Davy may think it worth while to have such apparatus constructed as I have employed and compare results. I would recommend however when you are [word illegible] to have the wires 6 feet long instead of 4 feet. I have no doubt that you would find more accurate results. The set screws I have mentioned may be very well dispensed with and then the upper rather is as simple and as little expensive as can be desired.

You will perceive that I have entered int<<o>> no mechanical illustration of the method made, by which the tangential force I have assumed is produced, and I have no reference therefore to the currents. I do not myself see why we should not as readily admit of a tangential force between two particles of fluid, as an attractive force; we have unquestionably as little knowledge of the modus operandi in one case as in the other and however the effect is produced, your experiments prove most decidedly that it actually exists and that is all I have assumed -

I remain Dear Sir Yours very truly | Peter Barlow

We shall expect to see you in the summer but you will probably see me first at the RS

Address: Mr M Faraday | Royal Institution | Albermarle Street