Michael Faraday, His Life and Work

by Silvanus P. Thompson

He gave four afternoon lectures at the Royal Institution and five Friday evening discourses. These were on optical deceptions, on light and phosphorescence, being an account of experiments recently made by Mr. Pearsall, chemical assistant in the Institution; on oxalamide, then recently discovered by M. Dumas; on Trevelyan’s experiments about the production of sound by heated bodies; and on the arrangements assumed by particles upon vibrating surfaces.

In 1832 he gave five Friday evening discourses, four of which related to his own researches. In August he entered upon the third series of “Experimental Researches in Electricity,” which was devoted to the identity of electricities derived from different sources, and on the relation by measure of common [i.e. frictional] and voltaic electricity. He did not like any doubt to hang about as to whether the electricity obtained from magnets by induction was really the same as that obtainable from other sources. Possibly he had in his mind the difficulties which had arisen thirty years before over the discoveries of Galvani and Volta, when it was so far doubted whether the electricity in currents from piles and batteries of cells was the same as the electricity evoked by friction, that the distinctive and misleading name of “galvanism” was assigned to the former. He commented on the circumstance that many philosophers—and he included Davy by name in an explicit reference—were vainly drawing distinctions[40] between electricities from different sources, or at least doubting whether their identity were proven. His first point was to consider whether “common electricity,” “animal electricity,” and “magneto-electric currents” could, like “voltaic electricity,” produce chemical decompositions. He began by demonstrating that an ordinary electric discharge from a friction machine can affect a suitably disposed galvanometer. One of his instruments of sufficient sensitiveness was surrounded by an enclosing cage of double metal foil and wire-work, duly connected to “earth,” so as to render it independent of all disturbances by external electric charges in its neighbourhood. His “earth” for this purpose consisted of a stout metal wire connected through the pipes in the house to the metallic gas-pipes belonging to the public gas works of London, and also with the metallic water-pipes of London—an effectual “discharging train.” He used a friction electric machine with a glass plate 50 inches in diameter, and a Leyden-jar battery of fifteen jars, each having about 84 square inches of coated glass. This battery of jars was first charged from the machine and then discharged through a wet thread four feet long, and through the galvanometer to earth viâ the “discharging train.” Having by this means satisfied himself that these electric discharges could deflect a galvanometer, whether through the wet thread, a copper wire, or through water, or rarefied air, or by connection through points in air, he went on to the question of chemical decomposition. Dipping two silver wires into a drop of solution of sulphate of copper, he found that one of them became copper-plated by the electricity that was evolved by 100 or 200 turns of the disc machine. He bleached indigo, turned starch purple with iodine liberated from iodide of potassium, exactly as might have been done by a “volta-electric current” from a battery of cells. He also decomposed water, giving due recognition to the antecedent experiments of Van Troostwyk, Pearson, and Wollaston.

IDENTITY OF ELECTRICITIES.

In the paper which he drew up he compares these results with others made with electric discharges from an electric kite and with those of the torpedo and other electric fishes. He recapitulates the properties of magneto-electricity and the proofs now accumulating that it can decompose water. He drew up a schedule of the different effects which electricity can produce, and of the different sources of electricity, showing in tabular form how far each so-called kind of electricity had been found to produce each effect. The conclusion was that there is no philosophical difference between the different cases; since the phenomena produced by the different kinds of electricity differ not in their character but only in degree. “Electricity, whatever may be its source, is identical in its nature.” On comparing the effects produced by different discharges, he concludes that “if the same absolute quantity[41] of electricity pass through the galvanometer, whatever may be its intensity, the deflecting force upon the magnetic needle is the same.” He was then able to go on to a quantitative comparison between the “quantity” of electricity from different sources, and came to the conclusion that both in magnetic deflection and in chemical force the current of electricity given by his standard battery for eight beats of his watch was equal to that of the friction machine evolved by thirty revolutions; further, that “the chemical power, like the magnetic force, is in direct proportion to the absolute quantity of electricity which passes.”

ELECTRO-CHEMICAL WORK.

This series of researches was published in January, 1833. In April of the same year he sent to the Royal Society another paper—the fourth series—on electric conduction. It arose from the surprising observation that, though water conducts, ice acts as a complete non-conductor. This led to an examination of the conducting power of fusible solids in general. He found that as a rule—excepting on the one hand the metals, which conduct whether solid or liquid, and on the other hand fatty bodies, which are always non-conductors—they assume conducting power when liquefied, and lose it when congealed. Chloride of lead, of silver, of potassium, and of sodium, and many chlorates, nitrates, sulphates, and many other salts and fusible substances were found to follow this rule. All the substances so found to act were compound bodies, and capable of decomposition by the current. When conduction ceased, decomposition ceased also. An apparent exception was found in sulphide of silver, which, when heated, acquired conducting powers even before it assumed the liquid state, yet decomposed in the solid state. This led him on to study electro-chemical decompositions more closely. Here he was following directly in the footsteps of his master Davy, whose discovery of the decomposition of potash and soda by the electric current had been one of the most prominent scientific advances resulting from the invention of the voltaic cell. The fifth series of researches, published in June, 1833, embodies the work. He first combats the prevailing opinion that the presence of water is necessary for electro-chemical decomposition; then analyses the views of various philosophers—Grotthuss, Davy, De la Rive, and others—who had discussed the question whether the decompositions are due to attractions exercised by the two poles of the electric circuit. This he contests in the most direct manner. Already he has reason to believe that for a given quantity of electricity passed through the liquid the amount of electro-chemical action is a constant quantity, and depends in no way on the distance of the particles of the decomposable substance from the poles. He regards the elements as progressing in two streams in opposite directions parallel to the current, while the poles “are merely the surfaces or doors by which the electricity enters into or passes out of the substance suffering decomposition.”

Amongst the laboratory notes of this time are many which were never published in the “Experimental Researches,” or of which only brief abstracts appeared. Some of these are of great interest.

Here is one literally transcribed:—

26 Feb. 1833.

Chloride Magnesium.—When solid and wire fuzed in non-conductor—When fuzed conducted very well and was decomposed A and P Pole much action and gas—chlorine? At N Pole Magnesium separated and no gas. Sometimes Magnesium burnt flying off in globules burning brilliantly. When wire at that pole put in water or white M A [muriatic acid] matter round it acted powerfully evolving hydrogen and forming Magnesia; and when wire and surrounding matter heated in spirit lamp Magnesium burnt with intense light into Magnesia. VERY GOOD EXPT.