Jean de Luc was a noted Swiss natural philosopher from 1773 to 1817. He made England his home and became reader to Queen Charlotte. In 1809, he presented the operative action of the Volta pile to the Royal Society of London, and separated its chemical action from its electrical action.
He also built his own version of a pile. Extensive dry pile experiments were performed in England between 1806 and 1811 by J.A. de Luc, a natural philosopher of Swiss extraction. Appointed reader to Queen Charlotte, the consort of George III, he had a long career as an experimenter and had travelled widely. Apart from having been for a time professor of philosophy and geology at Gottingen, he lived in Berlin, Hanover and Brunswick, before finally settling at an advanced age in London.
The results of his experiments communicated to the Royal Society were not published in their Philosophical Transactions. He expanded his account and added some new results for papers to be printed in A Journal of Natural Philosophy, Chemistry, and the Arts [Nicholson's Journal] in 1810. The reason why his papers were not printed in the journal of the Royal Society de Luc makes clear in a letter he addressed to the editor. In 1806 he deposited in the library of the Royal Society two papers written in Berlin and printed in Paris. The second one entitled Trait elementaire sur le Fluide electro-galvanique deals de Luc’s ideas on the properties of the voltaic pile.
His analysis was left unfinished because of geological studies undertaken in various parts of Germany, so he took the pile up again on his return to England. What spurred him on was Humphrey Davy’s Bakerian lecture on “some chemical agencies of electricity” which contained observations already disproved by de Luc in his paper deposited in the Society’s library. This resulted in a new paper being presented to the Royal Society on May 30, 1808. On being told that his paper was too long to be read at a meeting, he asked Joseph Banks for its return in early 1809 so that he could shorten it. By suppressing in his new account all the experiments that contradicted Davy, he managed to reduce it by twenty-three pages, but his less controversial version, presented to the Society on February 25, 1809, fared no better.
On March 7, de Luc presented a new paper “On the Electric Column, and Aerial Electroscope”, but the door remained firmly closed. The following summer he received a letter signed by Davy that “the Committee of Papers, although they did not think it proper to publish my papers at present, had directed that they be deposited in the Archives of the Society”. De Luc must have felt that the intention was to suppress his results that seemed to be contrary to Davy. A request by de Luc for the return of his drawings, so that he could get them engraved for papers to be published elsewhere, elicited no response. At the same time he felt that his “electric column” was no longer a novelty as he had shown it to Davy and many other “experienced philosophers” from July 1808. Nicholson responded in a footnote to de Luc’s letter that “he will print the learned author’s communications which my duty as a Journalist, and the nature and importance of those writings, may demand”.
In the first two papers in this series he described his “dissection” of the “galvanic pile”. Small tripods of thin brass wire were interposed between the units of the pile in three different ways. They were either placed between the zinc and silver plates (first dissection), between the silver plate and the wet cloth (second dissection), and finally between the zinc plate and the wet cloth (third dissection). Only in the case of the ‘uninterrupted’ pile (that is when de Luc did not separate the units or pairs by brass tripods) and in the first dissection were shocks felt and did chemical action take place (such as a litmus solution changing colour). The second dissection produced electrical, but no chemical effects and the third neither effect. He furthermore observed that only in the first dissection did the zinc plates show signs of corrosion (“oxidation”).
De Luc concluded that it was through the process of oxidation that the electric fluid experienced a change in nature, and it was this change that made the electric fluid capable of bringing about further chemical action (as shown by the effects exhibited). In order to ascertain if a liquid was essential, de Luc made up a pile with pieces of cloth not moistened, and he found the electrical effects still present but in a weaker state. This induced him to conduct a series of experiments, trying out different animal and vegetable substances between the metal pairs instead of the moistened cloth.
His preferred method was writing paper. His pile was made up of discs of sheet zinc and Dutch gilt metal separated by paper in such a way that the gilt side of the paper was in contact with the zinc. The discs were pressed together in a glass tube by a brass cap and screw connected at each end to a wire. He found that his apparatus had the same “electrical indications” as the common voltaic pile, but that it produced no chemical effects, nor was any oxidation of the zinc observed, even after protracted action. He concluded that what he had constructed was a “kind of perpetual electrical machine” in which the opposite electrical states perpetually exist, without any new excitement.
To distinguish it from the conventional voltaic pile, he proposed to call it the “Electric Column”. The column de Luc presented to the Royal Society was made up of 300 pairs. It showed identical effects to those produced by the frictional electrical machine, by applying all the standard tests for determining electricity. The dry pile’s intensity was measured with a gold leaf electroscope. When the intensity was sufficiently high, the gold leaf struck the side of the electroscope and then immediately collapsed as its charge was neutralized (earthed). The gold leaf was quickly recharged by the column and would thus strike the sides of the electroscope more or less continuously.
De Luc found that the speed of striking depended on several factors: the number of pairs in a column, the number of columns connected in the circuit, temperature and humidity. This led to his “aerial electroscope” (Figures 1 and 2) arranged in such a way that a pith ball covered with gold leaf and suspended from a silk thread was made to oscillate between two spheres connected to the poles of the pile.
In another version (similar to the specimen still active in the Clarendon Laboratory) he replaced the spheres by two brass bells, so that the rate of striking could be heard. This modification (Figure 2) was probably suggested by B.M. Forster (1764-1829), whose inspiration could well have been the “electric chimes”, a popular eighteenth century didactic device sold as an accessory in the electrostatic kits.
De Luc observed that the variations in the rate of striking of the dangling bob were according to the state of the atmosphere. It was reported that with a column of 20,000 pairs of silver, zinc and double discs of paper, a continuous ringing was maintained for more than two years. With this version he obtained sparks, and a Leyden jar was charged in ten minutes with sufficient electricity to produce shocks and to fuse an inch of thin platinum wire.
Over the next few years there was a flurry of activity repeating de Luc’s observations, in particular those relating to the apparent influence of the weather on the activity of the dry pile. Notable among these minor players were B.M. Forster, his unrelated namesake Thomas Foster and Thomas Howldy. A Mr J. Tatum suggested that the chief cause for variations in the ball’s oscillations was not moisture but owing to the increased temperature of the atmosphere.
There was also a brief exchange of papers between de Luc and Francis Ronalds (1788-1873) on this matter in 1814, which appeared to have been based on a misunderstanding. As it happened, both identified two different sources of the effect of humidity on the pile. Firstly, a certain degree of moisture was needed inside the column (that is of the “conducting” material interposed between the metals) for the pile to work. Variations in this moisture content affected the pile’s degree of activity. Secondly, the pile was affected by another source of moisture as indicated by the behaviour of the electrometer connected to the pile. Moisture deposited onto the outer glass tube, containing the column of the dry pile, reduced its electrical tension, as part of this was leaked away by the conducting film of moisture. The exchange was terminated by Ronalds with a rhetorical question about the cause of the dry pile’s electrical action: “Mr De Luc’s valuable experiments and observations lead to the conclusion that the presence of water, or some conducting fluid, in the substances which have been hitherto interposed between the metals, is necessary to the accumulation of electricity.
Whether this effect is occasioned by the presence of water only, because a conducting fluid is essential? Which I take to be the opinion; whether water acts merely as a conductor, offering in some unknown electric relation from the metals? Which I imagine to be that of Mr Singer; or, Whether any kind of decomposition is necessary are questioned not yet determined; but I have no doubt the researches of those intelligent gentlemen will contribute very materially to elucidate the subject.” George John Singer (1786-1817) in an admirable summary on the dry pile in his Elements of Electricity and Electrochemistry, published in 1814, reached the same conclusions as de Luc about the difference in action between the voltaic and the dry pile:
“It therefore appears indispensably necessary to the chemical power of the Voltaic apparatus, that a liquid be interposed between each pair of its plates, whilst for the pure electrical effects, the only condition appears to be the association of the two metals; and the connexion of the different pairs, by some conductor that does not interfere with the electro-motive power.” Singer’s intention was to test this by means of a very powerful dry pile consisting of 60,000 pairs, but this was not ready when his book was published.