Julius Plücker to Faraday   9 January 1859

My dear Sir!

I thank you, Sir, for your last kind letter of the 27th of July¹, and would not answer it without co<line>municating to you some new results regarding the electric discharge through gaz vacua. I had observed already the double current - mentioned in your letter - which is produced in Gassiots tube, if only one tin-cover is touched by one wire of Ruhmkorff’s coil. I had made use of it in order to confirm the theoretical views, contained in my paper of the 15th of July². I closer examined double currents in a recent paper³ in order to get analogies with the negative light, constituting the magnetic curves and surfaces. I think it probable that this light, starting from its electrode returns to it in the same way.- If the current find resistance of any description in a longer evacuated tube it partly returns on is own way (will it not be the same in very long isolated copper wires?).

But by far the principal object of my last longer paper⁴ (sent before Christmas, to Poggendorff) are the curious appearances of positive light which are as characteristic as those of negative light, already described. Let me try to give in a few words a general idea of them. From each of two electrodes entering an evacuated sphere of glass starts a peculiar kind of light. Even if the distance of the electrodes be a few millimetres only, there is produced no current; the double light, filling all the sphere is separated by the magnet. Suppose, for instance, an airvacuated sphere, through which are conducted two platina wires, equally distant from the centre and directed perpendicularly to each other. Put the sphere upon the two ironpieces of the Electromagnet, let the negative wire AB be vertical, the positive one axial; let the south polarity (the polarity of the North of the Earth) be behind. Then all the surface of the axial and vertical cercle passing through AB will be illuminated with beautifull violet light, while the reddish yellow positive light is concentrated in E the midst of CD and from E it moves in a fine spiral toward AB. Between the co<line>mencement of the spiral and the positive wire CD, there is a small dark interval. The spiral does not reach AB.

After having changed the polarity of the magnet, the appearance of the illuminated circle will not change, but instead of the single spiral, you will get two spirals, starting from the two ends C and D, both turned in the same way, but in opposite way as before. After a co<line>mutation of Ruhmkorff’s coil the phenomena totally change.

In my paper I described a great number of similar phenomena, especially with regard to the propagation of positive light. In admitting an electric particle - sit venia verbi ⁵ - starting from the different points of the positive wire towards the negative one and applying to its movement the well known law of the action of the Magnet on a moved electric particle - I became enabled to predict all phenomena observed, regarding the concentration of the positive light in certain points of its wire, as well as the curves, which it follows towards the negative wire. With other words, the same laws, which determine the action on a formed current are applicable to its formation.

In our case the positive light goes all the way towards the negative wire; here the combination of both electricities takes place, accompanied with chemical action and production of heat. The magnetic light, which has nothing to do with the current itself, may probably take its origine from these same sources.

If the air be less rarefied all changes. Accidentally my above mentioned sphere got a fissure through which air entered most slowly. The electric light opposed by an by a greater resistance to the magnetic action. The magnetic light of the circle AB disappeared, the positive light of the spirals became better defined and, at a certain period, the spiral above indicated by a few lines had a most striking resemblance with Donati’s⁶ comet⁷.

The indications of the nature of the electric discharge given to me by the magnet seems now, for the greater part, to be exhausted. Its analysis by the prism remains rather unexplorated. I would be happy if I could myself show to you the spectra of pure gazes, presented by the positive light on its way to the negative wire. The number of those spectra, observed by myself, is always increasing. The spectrum of mercurial vapour (at ordinary temperature) is the most brilliant and the most instructive too (The light in my tube of Gassiot is produced by this vapour).

The discontinuity of the index of refraction in the gaz-spectra is very extraordinary. If you observe through a telescope a narrow aperture illuminated by electric hydrogene-light, you will see a red band, we may suppose under an angle of 3’-15’. (For my researches I wanted exact measures of angles)[.] In interposing a prism of flint, you will observe under the same angle on a dark ground three fine bands, the first one is red, the second bluish green, the third violet. The second band is distant from the first about 3°, the third about 5°. Hence you deduce three different absolute indices for the electric hydrogene-light.

The pure gaz-spectra are only obtained by the current of positive light through most rarefied vacua. The light of the negative wire gives the spectrum of the gaz, mixed with bands depending upon the metal of the wire. &c &c &c -

Poggendorff was not so kind to send me copies of my paper, I am not able therefore to send some to England. I am anxious to know Gassiots new results, perhaps your own, if the seducing power of the object be great enough. Excuse me if I am too long.

Pray, present my respects and compliments to Mad. Faraday and believe myself with all my heart

Yours | Plücker

Bonn 9th of Jan. | 1859.