Julius Plücker to Faraday   10 August 1849

Bonn, | Aug. 10th, 1849

Dear Sir!

I feel myself very much obliged to you for having proposed me a member of the Royal Institution¹, and find no words to express my thanks in a proper way; but believe me Sir the kindness you showed to me on several occassions gave to me the greatest satisfaction I ever felt in my scientific career.

Permitt me to offer to you a “Resumé” of all my researches on Magnetism til[l] July 1849. Since my last letter I had scarcely any time to continue them. I found only crystals of Oxide of Tin showing a very strong magnetic polarity in the direction of their single axis; they were directed very well by the Earth. Then I examined most attentively the sulfate of iron. The line attracted by the poles of the Magnet is not perpendicular to the cleavage planes but makes with them an angle of 75˚. According to that you will observe that a piece of such a crystal, bounded by cleavage planes, points differently, when turned round its magnecrystallic axis, this axis being allways horizontal. The difference is measured by an angle of 15˚ on both sides. The line attracted by the poles is one of the mid[d]le lines between the optic axes (which include an angle of 90˚), the other one being not at all affected by the Magnet. In this (exceptional) case the resulting effect cant be deduced from the attraction of both the optic axes. Therefore I inquired, if the action may directly depend on the distribution of the Ether within the crystals, all the lines of less elasticity being attracted, the lines of greater elasticity repelled by the poles. But this law does not hold. Therefore new investigations only may give the true and complete law of nature.

I cut out of a very nice crystal of sulphate of iron a cube, two surfaces of which were perpendicular to the mid[d]le line attracted by the poles. By a sensible balance I found no difference in the magnetic attraction whatever a surface might be put on the approached poles of the Magnet. This result, fully according to your experiments, appears to me very strange: the directing power of the mid[d]le line being in this case so very strong.

These last days I tried again to prove that there is a diamagnetic polarity. The mutual action between magnetised iron being many thousand times stronger than that of magnetised iron on diamagnetic bismuth, you may never expect to see any mutual action between two pieces of diamagnetised bismuth. Such an action must be many - many million times weaker. But if you give to a piece of bismuth being acted upon by a magnet and suspended within a copper wire, by means of a current sent t[h]rough this wire alternately in opposite direction[s], a new diamagnetic polarity, the repulsion may be altered in the ratio of the intensity of the diamagnetic polarity, given to the bismuth by the Magnet, to to [sic] the intensity of that polarity, altered by the current. I[n] this way I succe[e]ded to show by means of the balance that a cylindre of bismuth obtained by the wire a magnetic polarity opposite to that which a magnetic body would obtain under the same conditions. But the action is very weak and I must before I may pronounce on this important point, repeat the same experiment in a varied way.

Being elected by the University of Bonn a deputy to the deliberations on the Universities’s reform, which will take place at Berlin by ordre of the Prussian government in the month of September, I am not able to accept the kind invitation from Birmingham². But I hope tis not the last time I crossed the Channel.

I had the pleasure to see Prof. Wheatstone here at Bonn, and was exce[e]dingly glad to learn from him, you were now of very good health.

Most truly | Yours | Plücker