Faraday to Arthur-Auguste De La Rive   7 March 1854¹

Royal Institution | 7 March 1854

My dear Friend

Your question “whether I have ever succeeded in producing induction currents in other liquids than mercury or melted metals, as for instance in acid or saline solutions?”² has led me to make a few experiments on the subject³, for though I believed in the possibility of such currents, I had never obtained affirmative results: I have now procured them, and send you a description of the method pursued. A powerful Electro magnet of the horseshoe form, was associated with a Grove’s battery of 20 pairs of plates. The poles of the battery were upwards, their flat end faces being in the same horizontal plane; they are 3.5 inches square and about 6 inches apart. A cylindrical bar of soft iron 8 inches long and 1.7 in diameter was employed as a keeper or submagnet: the cylindrical form was adopted, first, because it best allowed of the formation of a fluid helix around it; and next, because when placed on the poles of the magnet and the battery connexions made and broken, the magnet and also the keeper rises and falls through much larger variations of power and far more rapidly than when a square or flat faced keeper is employed; for the latter if massive has, as you know, the power of sustaining the magnetic conditions of the magnet in a very great degree when the battery connexion is broken. A fluid helix was formed round this keeper having 12 convolutions, and a total length of 7 feet; the fluid was only 0.25 of an inch in diameter, the object being to obtain a certain amount of intensity in the current, by making the inductive excitement extend to all parts of that great length, rather than to produce a quantity current by largeness of diameter, i.e by a shorter mass of fluid.

The helix was easily constructed by the use of 8.5 feet of vulcanized caoutchouc tube, having an internal diameter of 0.25 & an external diameter of 0.5 of an inch: such a tube is sufficiently strong not to collapse when placed round the iron cylinder. The 12 convolutions occupied the interval of six inches, & two lengths of 9 inches each constituted the ends. This helix was easily and perfectly filled, by holding it with its axis perpendicular, dipping the lower end into the fluid to be used & withdrawing the air at the upper; then two long clean copper wires 0.25 of an inch in diameter, were introduced at the ends, and being thrust forward until they reached the helix, were made secure by ligaments, and thus formed conductors between the fluid helix and the Galvanometer. The whole was attached to a wooden frame so as to protect the helix from pressure or derangement when moved to & fro. The quantity of fluid contained in the helix was about 3 cubic inches in the length of 7 feet. The Galvanometer was of wire 0.033 of an inch in diameter and 164 feet in length, occupying 310 convolutions: it was 18 feet from the magnet & connected with the helix by thick wires, dipping into cups of mercury. It was in the same horizontal plane with the magnetic poles & very little affected by direct action from the latter.

A solution formed by mixing 1 vol. of strong sulphuric acid & 3 of water was introduced into the helix tube, the iron keeper placed in the helix, & the whole adjusted on the magnetic poles in such a position, that the ends of the copper connectors in the tube were above the iron cylinder or keeper, and were advanced so far over it as to reach the perpendicular plane, passing through its axis: in this position the lines of magnetic force had no tendency to excite an induced current, through the metallic parts of the communication. The outer ends of the copper terminals were well connected together & the whole left for a time, so that any voltaic tendency due to the contact of the acid & copper might be diminished or exhausted: after that the copper ends were separated & the connexions with the Galvanometer so adjusted, that they could be in an instant either interrupted, or completed, or crossed, at the mercury cups. Being interrupted, the magnet was excited by the full force of the battery & thus the direct magnetic effect on the Galvanometer was observed; the helix had been so arranged, that any current induced in it should give a deflection in the contrary direction to that caused directly by the magnet; that the two effects might be the better separated. The battery was then disconnected & when the reverse action was over, the Galvanometer connexions were completed with the helix; this caused a deflection of only 2˚ due to a voltaic current generated by the action of the acid in the helix on the copper ends: it shewed that the connexion throughout was good, and being constant in power, caused a steady deflection, and was thus easily distinguished from the final result. Lastly the battery was thrown into action upon the magnet, and, immediately the galvanometer was deflected in one direction, & upon breaking battery contact, it was deflected in the other direction, so that by a few alternations, considerable swing could be imparted to the needles. They moved also in that particular manner, often observed with induced currents, as if urged by an impact or push at the moments when the magnet was excited or lowered in force; and the motion was in the reverse direction, to that produced by the mere direct action of the magnet. The effects were constant. When the communicating wires were crossed, they again occurred, giving reverse actions at the galvanometer. Further proof that they were due to currents induced in the fluid helix, was obtained, by arranging one turn of a copper wire round the iron core or keeper in the same direction as that of the fluid helix, and using one pair of plates to excite the magnet; the induced current caused in the copper wire was much stronger than that obtained with the fluid, but it was always in the same direction.

After these experiments with the highly conducting solution, the helix was removed, the dilute acid poured out, a stream of water sent through the helix for some time, distilled water then introduced and allowed to remain in it awhile, which being replaced by fresh distilled water, all things were restored to their places as before & thus a helix of pure water submitted to experiment. The direct action of the magnet was the same as in the first instance but there was no appearance of a voltaic current, when the galvanometer communications were completed; nor were there any signs of an induced current upon throwing the magnet into & out of action. Pure water is too bad a conductor to give any sensible effects with a Galvanometer & magnet of this sensibility & power.

I then dismissed the helix, but, placing the keeper on the magnetic poles, arranged a glass dish under it & filled the dish with the same acid solution as before; so that the liquid formed a horizontal disc 6 inches in diameter nearly, an inch deep & within 0.25 of an inch of the keeper; two long clean platinum plates dipped into this acid on each side of the keeper and parallel to it, and were at least five inches apart from each other; these were first connected together for a time, that any voltaic tendency might subside, and then arranged so as to be united with the galvanometer when requisite as before. Here the induced currents were obtained as in the first instance, but not with the same degree of strength. Their direction was compared with that of the current induced in a single copper wire passed between the fluid and the keeper, the magnet being then excited by one cell, & was found to be the same. However, here the possibility exists of the current being in part or altogether excited upon the portions of the wire conductors connected with the platinum plates; for as their ends tend to go beneath the keeper & so into the circuit of magnetic power formed by it and the magnet, they are subject to the lines of force in such a position, as to have the induced current formed in them; and the induced current can obtain power enough to go through the liquid, as I shewed in 1831⁴. But as the helix experiment is free from this objection, I do not doubt that a weak induced current occurred in the fluid in the dish also.

So I consider the excitement of induction currents in liquids not metallic as proved; and as far as I can judge, they are proportionate in strength to the conducting powers of the body in which they are generated. In the dilute sulphuric acid, they were of course stronger than they appeared by the deflection to be; because they had first to overcome the contrary deflection which the direct action of the magnet was able to produce: the sum of the two deflections in fact expressed the force of the induced current. Whether the conduction by virtue of which they occur is electrolytic in character or conduction proper I cannot say. The present phenomena do not aid to settle that question, because the induced current may exist by either one or the other process. I believe that conduction proper exists and that a very weak induction current may pass altogether by it, exciting for the time only a tendency to electrolysis, whilst a stronger current may pass partly by it & partly by full electrolytic action.

I am My dear friend | Ever most truly yours, | M. Faraday

Prof. | Aug de la Rive | &c &c &c &c