My dear Sir
The object of Mr Joule’s investigation1 is to enquire into the correctness of a law announced some years ago by Lenz and Jacobi2 and expressed by Mr Joule in the following words:– ‘When two bars of iron of different diameters, but equal to one another in length, and surrounded with coils of wire of the same length, carry equal streams of electricity, the magnetism developed in the bars is proportional to their respective diameters.’3 Referring to Poggendorff’s Annalen vol. 61 page 262, I find the law as stated by its discoverers to be as follows:– ‘In solid cylinders of iron of equal lengths and upwards of ⅓ of an inch in diameter, the magnetism developed by currents of equal strengths, sent through spirals possessing the same number of coils, is proportional to the diameters of the cylinders.’4 Instead of taking the same number of coils Mr Joule takes equal lengths of wire and winds them close round bars of different diameters: the number of coils must be therefore unequal – the same length of wire will go less often round a thick bar than round a thin one. I do not see therefore how experiments made with such an arrangement can furnish any evidence as to either the truth or fallacy of the Law of Lenz & Jacobi.
Mr Joule also observes upon the first page that he is not aware of any experimental researches having been made, since the publication of the law, which are calculated either to confirm or to confute it. Many such experiments are recorded in the Continental Journals. Among those who have made such researches may be mentioned Müller,5 Hankel,6 von Feilitsch7 and Dub.8 The most recent result with which I am acquainted is; that the magnetism developed in iron cylinders by equal currents, passing through the same number of coils, is proportional to the square roots of the diameters of the cylinders. (See Pogg. Ann. vol. 90. page 248 &c.)9
In page 3 Mr Joule compares the effect of a magnetized bar upon a magnetic needle at the respective distances of 6 and 12 inches; and states that the action at the former distance is 2.98 times the action at the latter.10 In one of the experiments cited however the effect at 6 inches is more than 5 times the effect at 12 inches. The ratio 2.98 has nevertheless been used in all Mr Joule’s subsequent experiments for the purpose of reducing the actions at 12 inches to the actions at 6. I cannot help thinking that such reductions must be very uncertain.
Mr Joule also states that the magnetism of the smaller bars ‘is nearly proportional to the squares of their diameters’. One of these bars is ⅛ of an inch in diameter and another ¼ of an inch – hence the magnetism of the latter ought to be nearly 4 times that of the former. In one of the experiments cited however the magnetism of the latter is less than twice that of the former. Remarks of a similar nature might be made on the table at page 4.
I would beg to add that these observations of mine ought to be received with caution, for I confess that I have no clear comprehension of the purport of Mr Joule’s experimental arrangements. This may possibly be traced to a desire on his part to express himself with brevity, or possibly to my ignorance of the subject in question.
I am my dear Sir | yours very faithfully | John Tyndall
25th July 1855
RS RR/3/155
Mr Joule’s investigation: Faraday also criticised Joule’s paper (Faraday Correspondence, 4:3002). The paper was resubmitted some months later and was again refereed by Tyndall (letter 1181) and Faraday (Faraday Correspondence, 4:3067). Despite these criticisms Joule’s paper was published as ‘Introductory Research on the Induction of Magnetism by Electric Currents’, Phil. Trans., 146 (1856), pp. 287–95. Page numbers below refer to the published version, which bears the date 20 June 1855 (p. 294), indicating that the paper was not substantially revised in the light of the referees’ reports. A postscript (dated 21 December 1855) was, however, added containing further experimental results (p. 295).
Lenz and Jacobi: The Russian physicist Heinrich Friedrich Emil Lenz (1804–65) served as professor of physics and physical geography at the University of St Petersburg. Moritz von Jacobi (1801–74) was a German physicist and engineer who also worked in St Petersburg. The law as published by Lenz in ‘Ueber die Bestimmung der Richtung der durch Elektrodynamische Vertheilung erregten Galvanische Ströme’, Poggend. Annal., 31 (1834), pp. 483–94, on p. 485: ‘If a wire moves in the neighbourhood of a galvanic current or a magnet, the galvanic current is induced in it, the direction of which is such that it would have caused a motion of the wire that would have been exactly the opposite of the one given it, provided it was movable only along the line of motion’ (C. Jungnickel and R. McCormmach, Intellectual Mastery of Nature: Theoretical Physics from Ohm to Einstein, 2 vols (Chicago: University of Chicago Press, 1986), vol. 1, p. 137.
When two bars … diameters: Joule, ‘Introductory Research’, p. 287.
In solid cylinders … the cylinders: M. Jacobi and E. Lenz, ‘Ueber die Gesetze der Elektromagnete’, Poggend. Annal., 61 (1844), pp. 254–80, on p. 262.
Müller: The German physicist Johann Heinrich Jakob Müller (1809–75), who taught at the University of Freiberg and published extensively on optics, photography and magnetism, including ‘Ueber die Magnetisirung von Eisenstäben durch den Galvanischen Strom’, Poggend. Annal., 79 (1850), pp. 337–44.
Hankel: Wilhelm Gottlieb Hankel (1814–99) was professor of physics at the University of Leipzig who contributed a number of papers on electrical subjects, including thermo-electricity and atmospheric electricity.
von Feilitsch: Tyndall probably had in mind F. C. O. von Feilitzsch, ‘Ueber den Magnetismus elektrischer Spiralen von verschiedenem Durchmesser’, Poggend.
Annal., 79 (1850), pp. 564–8.
Dub: Julius Dub (1817–73), who pursued electrical research in Berlin.
See Pogg. Ann. vol. 90. page 248 &c: J. Dub, ‘Gestze der Anziehung der Elektromagnete’, Poggend. Annal., 90 (1853), pp. 248–61 and 436–56.
2.98 times the action at the latter: Joule, ‘Introductory Research’, p. 289.
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