Lives of the electricians

by William T. Jeans

The mirror has been used in different ways for the measurement of light. In 1850, Arago gave a description of his attempts to determine its velocity, but failing eyesight prevented him carrying out his full design. The subject was, however, taken up by M. Fizeau and M. Foucault, who employed steam power instead of clockwork to give motion to the mirror. By Foucault’s method a beam of light was reflected from a revolving mirror to a fixed concave mirror, and before it was reflected back again the revolving mirror had moved a sufficient space to enable him to compute therefrom the velocity of light. Fizeau’s method was simpler. He made a toothed wheel revolve with great rapidity, while a beam of light passed through one of the open spaces between the teeth, and fell upon a reflecting mirror at a considerable distance away. If the wheel were at rest, the beam would be reflected back through the same space by which it had entered; but the wheel being in rapid motion, the reflected beam would either fall on the next tooth which would prevent it passing through, or if the motion were increased, it would get through the next opening. A variety of tests like these has given the velocity of light as about 187,000 miles per second.

Professor Wheatstone also rendered memorable service in connection with the development of spectrum analysis. In a paper which he communicated to the Dublin meeting of the British Association in 1835, on “The Prismatic Analysis of Electric Light,” he expounded a discovery which has since led to useful results. Most metals, such as iron, copper, and platinum, when exposed to the gas flame, impart no colour; for that purpose they must be subjected to a higher temperature; and Professor Wheatstone found that the best way of attaining the requisite temperature was by the use of the electric spark. He found that a single electric discharge passed through a gold wire at once dissipated the metal into vapour. He also showed that by looking through a prism at the spark proceeding from two metallic poles, the spectra seen contained bright lines which differed according to the kind of metal employed. “These differences,” he said, “are so obvious that any one metal may instantly be distinguished from others by the appearance of its spark, and we have here a mode of discriminating metallic bodies more ready than chemical examination, and which may hereafter be employed for useful purposes.” Hofmann has well said that “within this fact a new mode of distinguishing bodies from each other lay folded, like the tree within the seed, awaiting evolution. The new line of research thus opened by Wheatstone with reference to bright lines produced by electric discharges, was pursued in a variety of directions by several observers. Foucault (1849), Masson (1851-55), Angström (1853), Alter (1854-55), Secchi (1855), Plückar (1858-59), Bunsen and Kirchhoff (1860), were successively engaged in this inquiry. It would exceed the limits of this sketch to minutely describe the phenomena presented by the spectra of the known metals, or to dwell on the infinitely minute quantities of substances found to be capable of producing the effect. The extreme delicacy of the new process is now a familiar fact; and it is equally well known that in using this method, the presence of one metal scarcely interferes with that of another. It would be out of place here to do more than simply mention the astronomical applications of spectrum analysis; such as, for example, the determination by its means of the composition of the solar atmosphere, in which M. Kirchhoff has proved, with a degree of probability approaching to certainty, the presence of several metals well known on this earth; amongst others potassium, sodium, calcium, iron, nickel, chromium, &c.” This delicate test has made it possible to detect the presence of the two hundred millionth part of a grain (in weight) of sodium, while by revealing bright lines not referable to any known body it has been the means of discovering five new metals—cæsium and rubidium by Professor Bunsen in 1860, thallium by Mr. Crookes in 1861, indium by Professors Richter and Reich in 1864, and gallium by M. Lecoq in 1875.

The year 1836 was distinguished in the history of electricity by the discovery of the constant battery of Professor Daniell. Early in that year Professor Daniell, of King’s College, announced in a letter to Faraday, that he had been led to the construction of a voltaic arrangement which furnished a constant current of electricity for any length of time, and had thus been able to remove one of the greatest difficulties which had hitherto obstructed those who had endeavoured to measure and compare different voltaic phenomena. This constant battery, which he improved in the spring of the same year, is still in general use. In it the zinc is placed in a semi-saturated solution of sulphate of zinc, and the copper in a saturated solution of sulphate of copper, the two solutions being separated by a porous earthenware partition. This battery furnishes a constant supply of electricity for weeks together.

Early in 1837 Professor Wheatstone publicly called attention to the capability of the thermo-electric pile as a source of electricity. Seebeck of Berlin discovered in 1822 that when different metals are soldered together and their junction heated, a current of electricity is generated; and Nobili and Melloni contrived on that principle the thermo-multiplier, an apparatus which indicates the effects of heat by the deflections of a needle on a scale, like a thermometer, the needle being moved by the electricity produced by the heat. But this means of producing electricity was better known for its delicacy than for its strength till Professor Wheatstone made some experiments—probably the first made in England—for the purpose of showing how the thermo-electric pile could be utilised as a source of electricity. In his account of these experiments he stated that “the Cav. Antinori, director of the Museum at Florence, having heard that Professor Linari, of the University of Siena, had succeeded in obtaining the electric spark from the torpedo by means of an electro-dynamic helix and a temporary magnet, conceived that a spark might be obtained by applying the same means to a thermo-electric pile. Appealing to experiments, his anticipations were fully realised. No account of the original investigations of Antinori had reached England in April, 1837; but Professor Linari, to whom he early communicated the results, published certain experiments and observations of his own on the subject in L’Indicatore Sanese for December 13, 1836.” The interesting nature of these experiments induced Professor Wheatstone to attempt to verify the principal results. For that purpose he used a thermo-electric pile consisting of 33 elements of bismuth and antimony formed into a cylindrical bundle ¾ of an inch in diameter, and 1⅕ in length. The poles of this pile were connected by means of two thick wires with a spiral of copper ribbon 50 feet in length and 1½ inch broad, the coils being well insulated by brown paper and silk. One face of the pile was heated by means of a red-hot iron brought within a short distance of it, and the other face was kept cool by contact with ice. Two short wires formed the communication between the poles of the pile and the spiral, and the contact was broken, when required, in a cup of mercury (a non-conductor) between one extremity of the spiral and one of these wires. Whenever contact was thus broken a small but distinct spark was seen. He added that Professors Daniell, Henry, and Bache assisted in the experiments, and were all equally satisfied of the reality of the appearance. At another trial the spark was obtained from the same spiral connected with a small pile of fifty elements, on which occasion Dr. Faraday and Professor Johnson were present, and verified the fact. By connecting two such piles together, so that similar poles of each were connected with the same wire, the spark was seen still brighter. He concluded by stating that such experiments supplied a link that was wanting in the chain of experimental evidence tending to prove that electricity, from sources however varied, is similar in its nature and in its effects; and that the effect thus obtained from the electric current originating in the thermo-electric pile might no doubt be easily exalted by those who had the requisite apparatus at their disposal, till it equalled the effect of an ordinary voltaic pile.

As Professor Wheatstone was not accustomed to write articles or to deliver lectures, it is not an easy matter to measure the extent of his knowledge at any particular time; but one more incident may be mentioned as indicating the range of his studies on electricity about this time. Between 1830 and 1835 William Snow Harris wrote several articles in the Nautical Magazine on the utility of fixing lightning conductors in ships. It was a popular impression then that pointed metal rods attracted lightning. Snow Harris contended, on the contrary, that damage to ships occurred not where good conductors were, but where they were not, and that such conductors could no more attract lightning than a watercourse could be said to attract water, which necessarily flowed through it at the time of heavy rains. He afterwards prepared a list of 220 ships of the British Navy which were struck and damaged by lightning between 1792 and 1846. In June, 1839, a committee of the Admiralty consulted Professor Wheatstone and Professor Faraday as to the safety of the continuous conductors advocated by Snow Harris. To that committee Professor Wheatstone stated that “it has been proved beyond all doubt that electricity follows the best conducting path which is open to it; and that when it finds a metallic road sufficient to conduct it completely, it never flies to surrounding bodies greatly inferior in conducting power. The experiments of M. de Romas, made in France, with the electrical kite, immediately after Franklin’s first attempt, might satisfy the most timid in this respect. Imagine, writes he to the Abbé Nollet, ‘that you see sheets of fire nine or ten feet long and an inch broad, which made as much or more noise than reports of a pistol. In less than an hour I had certainly thirty sheets of these dimensions, without counting a thousand others of seven feet and under. But what gives me the greatest satisfaction in this new spectacle is that the largest sheets were spontaneous, and notwithstanding the abundance of fire which formed them, they constantly followed the nearest conducting body. This constancy gave me so much security that I did not fear to excite this fire with my discharger, even when the storm was violent; and when the glass branches of the instrument were only two feet long I conducted wherever I pleased, without feeling the smallest shock in my hand, sheets of fire six or seven feet long, with the same facility as those of only six or seven inches.’ The wire of the kite was insulated, and the sparks were drawn by a metallic conductor held in the hand by means of an insulating handle, and communicating with the ground by a chain. The human body is known not to be one of the worst conductors; yet, because it was two feet further than a far more perfect one, it received none of the discharge, even though the conducting path were an interrupted one. The phenomenon to which the name of lateral explosion has been generally given was first observed by Henly, more than half a century ago, and has been subsequently experimented upon by Priestly, Cavallo, and more recently by Biot.” The committee attached the greatest weight to the opinion of Professor Wheatstone, which Faraday supported, and which was eventually adopted. Experiment and experience confirmed its accuracy.

At the time when he had attained such a recognised position as an electrician he was making progress in another field of electrical study in which he was destined to gain still greater eminence and to obtain more extensive and permanent results.

FOOTNOTES:

[6] The accuracy of Wheatstone’s experiment has been generally accepted; but, as Faraday said in 1838, “the velocity of discharge through the same wire may be greatly varied by circumstances.... If the two ends of the wire in Professor Wheatstone’s experiment were immediately connected with two large insulated metallic surfaces exposed to the air ... then the middle spark would be more retarded; and if these two plates were the inner and outer coating of a large jar, or a Leyden battery, then the retardation of that spark would be still greater.”

CHAPTER II.

“There is a certain meddlesome spirit which, in the garb of learned research, goes prying about the traces of history, casting down its monuments, and maiming and mutilating its fairest trophies. Care should be taken to vindicate great names from such pernicious erudition. It defeats one of the most salutary purposes of history, that of furnishing examples of what human genius and laudable enterprise may accomplish. For this reason some pains have been taken to trace the rise and progress of this grand idea (in the mind of Columbus); to show that it was the conception of his genius, quickened by the impulse of his age, and aided by those scattered gleams of knowledge, which fell ineffectually upon ordinary minds.”—Washington Irving.