WILCKE, JOHAN CARL

(b. Wismar, Germany, 6 September 1732; d. Stockholm, Sweden, 18 April (1796)

physics.

Like many of the Swedish savants of the eighteenth century, including Samuel Klingenstierna and Mårten Strömer, his physics professors at the University of Uppsala, Wilcke came from a clerical family. His father, Samuel Wilcke, the son of a Pomeranian shoemaker, had educated himself for the ministry with the aid of generous patrons, especially F. A. Aepinus, professor of theology at the University of Rostock, whose children he tutored. In 1739 Samuel was called to minister to the German-speaking community in Stockholm, where he spent the remainder of his life.

Wilcke received his secondary education at the German school associated with his father’s church. In 1750 he entered the University of Uppsala to prepare for the ministry. It was not theology, however, but mathematics and physics that aroused his interest; and for three terms he followed lectures on algebra, spherical trigonometry, mechanics, and experimental physics.

Hoping, perhaps, to save his son from science, Samuel Wilcke agreed to Johan Carl’s wish to study at Rostock. The elder Aepinus having died, Samuel counted on his former pupil, A. I. D. Aepinus, now holder of the Rostock chair of oratory, to urge the merits of the ministerial life. The scheme backfired. At Aepinus’s home, where Wilcke boarded, lived the rhetorician’s younger brother Franz, who, having rejected the family’s plan to make him a physician,¹ taught mathematics at the university. The bond of common interest and sympathy that soon developed between Franz Aepinus and Wilcke accelerated Wilcke’s drift from theology; and in 1753, when he matriculated at Göttingen, he no longer inscribed himself “theologus,” as he had at Rostock, but as “mathematicus.” Two years later A. I. D. Aepinus brought Samuel Wilcke to acquiesce in the fait accompli, perhaps made more palatable by the success of Franz, Aepinus, who in the spring of 1755 became the astronomer of the Berlin Academy of Sciences.

Electricity. Wilcke joined Franz Aepinus in Berlin, where he devoted his newfound freedom to the study of physics, particularly to the agitated question of the contrary electricities. Were Dufay’s vitreous and resinous electrifications differences only in degree, as Nollet insisted, or in kind, as the Franklinists claimed? Aepinus initially inclined toward Nollet, while Wilcke remained uncommitted, predisposed toward Franklin by certain experiments with the Leyden jar but arrested by Nollet’s “apparently unanswerable” demonstration of the permeability of glass.² To resolve these uncertainties, Wilcke repeated all the experiments urged on either side; he prepared an annotated translation of Franklin’s letters; and he found that, in most cases, Nollet’s objections rested on misinterpretations of obscure, imprecise, or abbreviated passages in Franklin’s work. As for Aepinus, he became an enthusiastic Franklinist when an experiment he designed to confirm Nollet failed.³

Not that Franklin’s theory was unexceptionable. In his doctoral dissertation, defended at Rostock in 1757, and again in notes to his edition of Franklin, Wilcke showed that absolute insulation did not exist, that any electric per se could act the part of glass in the Leyden experiment, that the charges on the two coatings of the jar are not quite equal, and that substances are not innately vitreous or resinous.⁴ This last point Wilcke owed to Canton, who had found that rough glass might be made minus or plus by rubbing with flannel or oiled silk. respectively. Recognizing that friction set up a competition for electrical matter, Wilcke hit on the idea of drawing up a winner’s list, the entries being so placed that a given one became positive (or negative) when rubbed by those placed beneath (or above) it. His sequence, the first triboelectric series, consisted of smooth glass, wool, quills, wood, paper, sealing wax, white wax, rough glass, lead, sulfur, and metals other than lead.⁵

The most important result of Wilcke’s Berlin period was the invention of the air condenser. Wilcke had consulted Aepinus–who had been studying the electricity of the tourmaline–about Franklin’s version of Canton’s induction experiments. Aepinus saw that the experimental arrangement amounted to an imperfect Leyden jar with air as dielectric; to check his insight he and Wilcke built a large air condenser (fifty-six square feet) that gave a shock comparable to that from a well-charged bottle. This demonstration threatened the already moribund theory of electrical atmospheres, which Franklin himself had not entirely discarded. While the repulsive force of the upper plate certainly reached the lower, its redundant electrical matter as certainly did not: for in that case the condenser, being shorted internally, could not have charged. Aepinus concluded for an instrumentalist theory of electricity, freely admitting action at a distance without specifying its cause. Indeed, he said, Franklinism must end in agnosticism: to save the simplest electrical phenomena, the particles of common matter must be supposed to repel one another at the same time that, according to the gravitational theory, they are mutually attractive.⁶

Wilcke did not embrace his friend’s teachings altogether. He tentatively accepted the most bizarre of the new postulates, the mutual repulsion of matter particles, in order to conquer the enigma of the repulsion between negatively charged bodies; but he continued to ascribe the reciprocal recession of positive bodies to the pressure of their atmospheres. In Wilcke’s asymmetric concept, positive atmospheres are material bodies, while negative ones are mere spheres of activity, spaces distorted by the presence of a deficient object.⁷ Several years later (1763) Wilcke resolved this asymmetry by accepting and even championing the dualistic theory of Robert Symmer, which replaced the Franklinist negative state, or absence of electrical matter, with the presence of a second electrical fluid [6 ].

The productive collaboration ended in 1757 when Aepinus left Berlin for St. Petersburg. Wilcke had also received a Russian offer but declined it when Klingenstierna contrived to find him a position in Sweden, a lectureship (which in 1770 became a professorship) in experimental physics at the RoyalSwedish Academy of Sciences. The position paid so poorly that Wilcke had to tutor for room and board, Not until 1777, when his salary had tripled, did he feel he could marry; he chose his housekeeper, Maria Christina Setterberg, who bore him no children to increase his expenses. Not until 1784, when he became secretary of the Academy, did his financial difficulties end. Some of Samuel Wilcke’s misgivings about his son’s career had beenwell taken.

Wilcke continued to work on electricity during his first year in Sweden [3 ]. His most characteristic efforts [4 ], on the location of charge in a dissectible plate condenser, anticipated the invention of the electrophorus; he observed that, having electrified the condenser, a plate could be removed, discharged, returned, grounded, and again removed, discharged, and so on, “without further electrification by the machine . . . as often as the trial is made.” Wilcke explained these effects as inductive in 1762, some thirteen years before Volta described similar experiments without explanation; when he learned of the electrophorus, Wilcke immediately supplied its theory [13 ]. He acknowledged Volta’s invention of a useful machine but rightly asserted priority in discovering its principle, a claim supported by most German-speaking electricians.⁸

Among Wilcke’s other electrical researches his lengthy studies of the tourmaline [7 ] and of cyclones and waterspouts [15 ] deserve mention. The former are distinguished by careful examination of a multitude of delicate cases that established the validity of Aepinus’s concept of electrical poles and corrected many previous errors of detail. The latter, although they do not in fact concern electricity, stemmed from Wilcke’s conjecture that cyclonic winds might be driven by atmospheric electricity. With his usual care he gathered all available data on waterspouts and compared them with the behavior of vortices and whirlpools generated in the laboratory. His knowledge of the phenomena was not superseded during the nineteenth century.

Heat Wilcke’s best-known work was his independent discovery of latent heat [11 ], which, he said, followed from a chance observation made early in 1772. Wishing to remove snow from a small courtyard and expecting that in obedience to Richmann’s law (which states that the temperature R of a mixture of two measures of water, m₁ and m₂ initially at temperatures T₁ and T₂, is R = [m₁T₁ + m₂T₂]/[m₁ + m₂]), hot water would melt more than its weight of snow, he was surprised to find the water of very little efficacy and concluded that the law did not hold for mixtures of ice and water. He therefore looked for a new rule. In a typical experiment Wilcke mixed hot water at temperature T and melting snow, measured the resultant temperature θ, and computed the difference between θ and R, the final temperature to be expected from Richmann’s law if water at zero degrees had been used in place of the snow.

In the simplest case, when all masses were equal. the mean loss of heat R – θ was 36 3/28; degrees; hence, as Wilcke concluded, it requires somewhat more than seventy-two degrees of heat to melt unit mass of snow at zero degrees.⁹ He observed that these seventy-twodegrees disappear or, as we would say, become latent, in liquefying the ice, and that lique faction occurs without change of temperature. Physically (according to Wilcke) the matter of heat, which, like Franklin’s electrical fluid, is made up of mutually repellent particles attracted by common matter, insinuates itself between contiguous ice particles, transforming them into water; further heating causes the water to expand and raises its temperature.¹⁰

These experiments probably owed less to chance than Wilcke represented. In 1769, while pursuing an old hobby, the study of the shapes of snow flakes and ice crystals, he had made the “paradoxical” observation that water cooled below zero degrees warms on freezing [10 ]. As Oseen observes, the melting of the courty ard snow with warm water was probably an attempt to study the paradox: it was an experiment, not an accident.¹¹

Wilcke returned to the problems of heat in the winter of 1780–1781, interrupting his study of waterspouts in order to follow up Joseph Black’s concept of specific heat as reported in J. H. Magellan’s Nouvelle théorie du feu élémentaire (1780). Wilcke had probably hit upon the same idea (although not the term) a few years earlier, perhaps in pursuing a note in Klingenstierna’s Inledning til naturkunnigheten (1747), a translation of Musschenbroek’s Elementa physicae. The note criticized Boerhaave’s opinion, approved by Musshenbroek, that at equal temperatures all bodies contain equal amounts of heat by volume. In experiments apparently done in the 1770’s, Wilcke showed that the sensible heats in bodies in thermal equilibrium were proportional neither to volume nor to mass: and, after a few false starts, he found how to measure relative heat capacities.¹² Immerse a mass of metal at temperature T in an equal mass of ice-cold water and record the resultant equilibrium temperature θ Next calculate by Richmann’s formula the amount of water w at temperature T that, when mixed with the same quantity of ice cold water (taken as unity for convenience), would yield the same resultant θ

Wilcke probably obtained w for gold and lead before 1780. After seeing Magellan’s book, he measured it for ten other substances [16 ]. Although his numerical results were not good (as in the experiments on latent heat he ignored the heat capacity of the calorimeter¹³), he understood that the w’s were the specific heat capacities that he and Black had sought. He also saw in them a further analogy between the properties of the matters of heat and electricity: for not only were they all subtle, elastic, and apparently weightless fluids, but each was retained in ponderable bodies by specific forces dependent upon the nature of the bodies [17 ].

Miscellaneous Researches. Between 1763, when he finished the electrical studies begun in Berlin, and 1772 Wilcke worked at terrestrial magnetism. He began by inventing a new declination compass[5 ] and immediately became its slave [14 ]; a few years later, about 1766, he designed a dipping needle that proved itself on a voyage to china [12 ]. Encouraged by its performance, he undertook the difficult task of selecting reliable data from conflicting published measurements of dip made with other instruments. The result, an important contribution [9 ], was a systematic isoclinal chart that showed a magnetic equator and indicated positions for the poles that approximated those obtained from mapping declination. In another important work [8 ], Wilcke showed that a soft iron needle may be magnetized naturally by placing it in the magnetic meridian, or artificially by setting it near a powerful lodestone; in either case the needle magnetized more readily if the discharge from a Leyden jar passed through it first. Wilcke explained that the discharge helped to rearrange the internal parts of the needle.

Like many leaders of Sweden’s eighteenth-century scientific renaissance, Wilcke had a taste and talent for applied science. He improved many standard instruments; the magnetic needles, the air pump, the micrometer, the barometer, the eudiometer. He also made suggestions for ventilating ships, for cooking under pressure (“Papin’s digester”), for life preservers, and – at the request of the government–for fortifying the harbor of Landskrona.

Wilcke was a dry, unsociable man, happiest when at work or when reading in the several non-scientific subjects that interested him: theology, travel, belles lettres, music. These qualities made his tenure as secretary of the Academy (1784–1796) a mixed success. A responsible and diligent bureaucrat, he kept up the Academy’s correspondence, publications, and records, and tried to maintain its high standards, as exemplified in his own scientific work. But he lacked the spark and influence of his predecessor, the astronomer Pehr Wargentin. Wilcke was not the man to win the Academy public support or to change its direction when, in the 1780’s, It fell into a decline that, however, did not approach bottom until after his time.¹⁴

As a physicist Wilcke is distinguished, apart from his substantive contributions, by his emphasis on measurement, exactness, and reproducibility of results. Although not a mathematical physicist in the modern sense, he insisted upon the utility of mathematics and mathematical formulations in experimental philosophy. In these emphases he was by no means unique or original, but he was one of the first physicists to demonstrate their fruitfulness in his own work.¹⁵

NOTES

1. J. C. Koppe, Jetzlebendes gelehrtes Mecklenburg, I (Rostock-Leipzig, 1783), 9–15.

2. F. U. T. Aepinus, Recueil de différents mémoires sur la tourmaline (St. Petersburg, 1762), 134; Wilcke, [2 ], intro., 388–389. The boldface numberals refer to items in the bibliography.

3. Aepinus, Recueil, 134–137; Wilcke, [2 ], 280–286, 348.

4. Wilcke, [2 ], 219–221, 271–272, 290, 308–309; and [1 ], 59–60, 81–83.

5. Wilcke, [1 ], 44–64.

6. Aepinus, Tentamen theoriae electricitatis et magnetismi (St. Petersburg, 1759), 5–7, 35–40, 75–83, 257–259; Wilcke, [2 ], 306–309.

7. Wilcke, [2 ], 221–224, 233–236, 262–263, 270–271, 307, 340–341.

8. G. C. Lichtenberg, Briefe, A. Leitzmann and C. Schüddekopf, eds., III(Leipzig, 1904), 203.

9. Let L be latent heat, M the mean of the experiments; then (since R = T/2), T – 2θ = 2M, and T – θ = L + θ whence L = 2M = 72 and 3/14 degrees. The value should be near 80.

10. Wilcke, [11 ], 105, 111; [16 ], 52.

11. Oseen, Wilcke, 156, 174–177.

12. This is an undated MS analyzed by Oseen, Wilcke, 232–234, 247–248.

13. McKie and Heathcote, Specific and Latent Heats, 86–87.

14. Lindroth, Historia, II, 20–26.

15. See Wilcke to W.C.G. Karsten, 1 July 1785. in Karsten, Physisch-chemische Abhandlungen, I (Halle, 1786), 118–119.

BIBLIOGRAPHY

I. Original Works. A bibliography of Wilcke’s printed work and a catalog of his scientific MSS held at the Royal Swedish Academy of Sciences are given in C. W. Oseen, Johan Carl Wilcke Experimental-fysiker (Uppsala, 1939), 369–391. The Academy also has much administrative and scientific correspondence from Wilcke’s secretaryship. Some scientific correspondence, notably that with C. W. Scheele, is printed in Oseen’s biography. All Wilcke’s scientific papers were published in Swedish in Kungliga Svenska vetenskapsakademiens handlingar (abbreviated below as Handl.) and translated into German in A. G. Kaestner, Der Kouml;nigl. schwedischen Akademie der Wissenschaften, Abhandlungen aus der Naturlehre (abbreviated below as Abh.); in the following bibliography the pages in Kaestner are given after the citation to the Handlingar.

Wilcke’s most important works are [1 ] his thesis, Disputatio physica experimentalis de electricitatibus contrariis (Rostock, 1757); [2 ] his ed. of Franklin’s letters, Des Herrn Benjamin Franklins Esq. Briefe von der Electricität . . . nebst Anmerkungen (Leipzig, 1758); [3 ] “Electriska rön och försök om den electriska laddningens och stötens åstadkommande vid flera kroppar än glas och porcellain,” Handl., 19 (1758), 250–282 (Abh., 20 , 241–268); [4 ] “Ytterligare rön och försök contraira electriciteterne vid laddningen och därtil hörande delar,” Handl., 23 (1762), 206–229, 245–266 (Abh., 24 213–253, 235–274); [5 ] “Beskrifning på en ny declinations-compass,” handl., 24 (1763), 143–153 (Abh., 25 , 154–164); [6 ] “Electriska försök med phosphorus,” handl., 24 (1763), 195–214 (Abh., 25 , 207–226): [7 ] “Historien om tourmalin,” Handl., 27 (1766), 89–108, and 29 (1768), 3–25, 97–119 (Abh., 28 95–113, and 30 3–26, 105–128): [8 ] “Afhandling om magnetiska kraftens upväckande genom electricitet,” Handl., 27 (1766), 294–315(Abh., 28 , 306–327); [9 ] “Försök til en magnetisk inclinations-charta,” Handl., 29 (1768), 193–225 (Abh., 30 , 209–237): and [10 ] “Nya rön om vattnets frysning til snö-like is-figurer,” Handl., 30 (1769), 89–111 (Abh., 31 , 87–108).

Also see [11 ] “Om snöns kyla vid smältningen,” Handl., 33 (1772), 97–120 (Abh., 34 93–116): [12 ] “Om magnetiska inclinationen, med beskrifning på tvänne inclinations-compasser,” Handl., 33 (1772), 287–306 (Abh., 34 , 285–302): [13 ] “Undersökning om de vid Herr Volta’s nya elettrophoro-perpetuo förekommande electriska phenomener,” Handl., 38 (1777), 56–83, 128–144, 216–234 (Abh., 39 , 54–78, 116–130, 200–216);

[14 ] “Rön om magnet-nålens årliga och dagelige ändringar i Stockholm,” Handl., 38 (1777), 56–83, 128–144, 216–234 (Abh., 39 , 54–78, 116–130, 200–216); [14 ] “Rön om magnet-nålens årliga och dagelige öndringar i Stockholm,” Handl., 38 (1777), 273–300 (Abh., 39 , 259–284); [15 ] “Försök til uplysning om luft-hvirflar och sky-drag,” Handl., 2nd ser., 1 (1780), 1–18, 83–102, 3 (1782), 3–35, 6 (1785) 290–307,and 7 ()1786), 3–20 (Neue Abh., 1 3–18, 81–97, 3 3–31, 6 , 271–286, and 7 3–27); [16 ] “Rön om eldens specifiska myckenhet uti fasta kroppar, och des afmätande,” Handl., 2 (1781), 49–78 (Neue Abh., 2 , 48–79; also Journal de physique, 26 [1785], 256–268, 381–389); and [17 ] “Rön om varmens spänstighet och fördeling, i anledning of ångors upstigande och kyla, uti förtunnad luft,” Handl., 2 (1781), 143–163 (Neue Abh., 2 146–164).

II. Secondary Literature. The standard biography is Oseen’s for Wilcke’s activities at the Academy, see also N. V. E. Nordenmark, Pehr Wilhelm Wargentin (Uppsala, 1939), and S. Lindroth, Kungliga svenska retenskapsakademiens historia 1739–1818 (Stockholm, 1967). A brief notice by Anna Beckman appears in S. Lindroth, ed., Swedish Men of Science (Stockholm, 1952), 122–130. For Wilcke’s work in general, see Oseen: on heat, see also D. McKie and N. H. de V. Heathcote, The Discovery of Specific and Latent Heats (London, 1935), 78–108; and on electricity, J. Priestley, The History and Present State of Electricity, 3rd ed. (London, 1775), I, 272–276, 358–362, and II, 35–37; and E. Hoppe, Geschichte der Elektrizität (Leipzig, 1884).

J. L. Heilbron