Townsend, John Sealy Edward
TOWNSEND, JOHN SEALY EDWARD
(b. Galway, Ireland, 7 June 1868; d. Oxford, England, 16 February 1957)
Townsend is best known for his research concerning the kinetics of ions and electrons in gases. The son of a college professor, he was educated at Trinity College, Dublin. He studied mathematics and physics, receiving his degree in 1890. After five years of teaching mathematics, in 1895 Townsend became one of the first outside research students to enter the Cavendish Laboratory under J. J. Thomson.
In 1897 Townsend made a direct determination of the absolute unit of charge using an original method, which “included practically all the ideas which were later used in accurate measurments of the charge.”1 Using Stokes’s law, Townsend measured the rate of fall of a cloud that had condensed on an electrified gas, which had been liberated in electrolysis and then bubbled through water. By February 1898 he published the unit of charge as 5 x 10–10 esu.2 In 1898 Townsend proved that the fundamental constant of electrolysis was equivalent to the charge carried by a gaseous ion whatever its mode of production. In that same year he also developed a method for determining the rate of ion diffusion indirectly using the ion mobility. By August 1900, the year of his election as Wykeham professor of physics at Oxford, Townsend had published a preliminary statement of his unique collision theory of ionization. Townsend had published a preliminary statement of his unique collision theory of ionization. Considering
Schematic current-voltage characteristic, for gaseous discharge at low pressure (c o . I mm Hg).
the ionization potential to be less than 15 volts instead of more than 150 volts, as was then commonly held, Townsend established that the motion of ions under the influence of an electric field was sufficient to form secondary ions in the gas. The ionization by collision was caused mainly by the “negative ions,” which Townsend considered “the same as the negatively charged particles which are given off when ultra-violet light falls on a zinc plate. It has been shown by Professor [J.J.] Thomson that the mass of these particles is 1/500 of the mass of a molecule of hydrogen.”3 The rate of secondary ionization by electrons was a function of the pressure and the applied voltage. His theory could adequately account for the anomalous increased in conductivity under ultraviolet light observed already in 1890 by Stoletow. By 1903, the year of his election as fellow of the Royal Society. Townsend had included the role of the positive ions in his collision theory. He developed an expression,4 ultimately containing his two well-known ionization coefficients α and γ, by which he could describe the Townsend discharge and also the breakdown or spark discharge (see Figure 1).
The collision principle was the basis of the 1908 particle detector of E. Rutherford and H. Geiger. Townsend had written to Rutherford that
. . . the dodge of multiplying small conductivity by collisions work very well. . . .The case of a wire inside a cylinder has been worked out by Kirkby.5
. . . You are certain to get unsteady effects if you try to multiply by too big a factor as the small variations produced in [the coefficients] α and β by variations of EMF or pressure have a large effect on the multiplier when it rises to the value of 500 or 1000.6
Kirkby and Townsend also quantitatively investigated electrochemical effects of the ionization of gases. Townsend studied the motion of electron swarms, noting that individual electrons may have random velocities much greater than the mean drift velocity of the swarm. He also noted that the mean free path of electrons in gases is energy-dependent. By the end of World War I Townsend was fifty, but during the subsequent two decades he averaged over two scientific papers a year and published five books. During the early 1920’s Townsend, independently of C. Ramsauer, discovered a new physical effect. He reported in 1924 the fact that the monatomic gases, especially argon and helium, seemed particularly transparent to low energy electrons, since these could traverse such gaseous media without “feeling” its presence. Ramsuer had observed the diminished dispersion for such slow-speed electrons compared with swifter ones. Although Townsend did not himself become involved with quantum theory, this Ramsauer Townsend effect7 was the analogue in gases of the results with solid-state targets obtained a few years later by C. J. Davisson and independently by G. P. Thomson, and it thus became important in the understanding of the wave nature of the electron. In 1941 Townsend was knighted in recognition of his many scientific contributions, and he died in his eighty-eighth year.
1. E. Rutherford, “The Development of the Theory of Atomic Structure,” in J. Needham and W. Pagel, eds., Background to Modern Science (Cambridge, 1938), 64–65.
2. This result compared favorably with that of R. A. Millikan, who, like Townsend, had avoided the expansion principle and who had closely approximated by 1911 the accepted value of 4.8 x 10-10 esu. Using an expansion chamber to form a cloud. J.J. Thomson published a result of 6.5 x 10-10 esu in December 1898.
3. J. Townsend, “The Conductivity Produced in Gases by the Motion of Negatively charged Ions,” in Philosophical Magazine1 (1901), 198–227. Considerable confusion could arise concerning Townsend’s term “negative ion,” for it could be taken as indicating a massive particle that had acquired a negative charge. However, whether he described the negatively charged particle as having passed from an ionic state into an electronic state (Electricity in Gases , p. 119) or as simply electrons (Electrons in Gases , p. 92), his negative ions were very small with respect to the molecule of hydrogen.
4. Townsend’s first expression was contained in “Some Effects Produced by Positive Ions,” in Elecrtrician50 (1903), 971.
The α and β were his first set of ionization coefficients, where α was the number of collisions or ion pairs produced per centimeter of path by an electron, and where β was the number of ion paris produced per centimeter by a positive ion. The number of electrons produced by the external radiation was “n0” “n” represented the total number of electrons arriving at the anode, and “a” was the distance between the electrodes. The dominant secondary effect of the positive ions. however, was not in gaseous collisions but was the release of secondary electrons at the cathode. In addition to α (the ionization coefficient representing the electrons released by other electrons colliding with the gas on their way to the anode), Townsend introduced his secondary emission coefficient γ which represented the ionizing electrons released at the cathode. The general expression accordingly became:
The sparking potential was a maximum value, and Townsend emphasized that the potential required for sustained discharge was normally significantly less. The conditions for breakdown were obtained on either expression by letting “n” approach infinity. For given Applied voltage and pressure the critical distance “d” for the spark discharge (the end of the region of Townsend discharge and the beginning of the region of the field-sustained discharge) was thus described on the earlier formulation byα=β exp (α-β) d, and on the later formulation by 1 = γ (exp αd – 1).
5. P. J. Kirkby, a research student of Townsend, “On the Electrical Conductivities Produced in Air by the Motion of Negative Ions,” in Philosophical Magazine, 3 (1902), 212–225.
Townsend published over one hundred papers and several books as listed by his biographer A. von Engel, in “John Sealy Edward Townsend, 1868–1957,” in Biographical Memoris of Fellows of the Royal Society, 3 (1957), 257–272; and in his notice for the Dictionary of National Biography; 1951–1960, pp. 983–985. A small collection of his correspondence exists at the Cambridge University Library Add. MSS 7653/T71–T89; but T. S. Kuhn has noted, in Sources for History of Quantum Mechanics (Philadelphia, 1967), p. 92b, that no extensive collection is likely to exist. See also A. von Engel, in Nature, 179 (1957), 757–758, and Maurice de Broglie, “Notice nécrologique sur Sir John Townsend,” in Comptes rendus hebdomadaires des séances de l’Académie des sciences244 (1957), 3105–3106. In A History of the Cavendish Laboratory (London, 1910), E. Rutherford in ch. 6 and C. T. R. Wilson in ch. 7 discussed the early work of Townsend. After over 40 years of continuous service at Oxford, at his retirement a notice of his work appeared in Nature, 157 (1946). 293. A valuable article is by C. A. Russell, in T. I. Williams, ed., A Biographical Dictionary of Scientists (London, 1969), 517–518. The work of Townsend concerning diffusion of ions in gases and also regarding the fundamental unit of charge is considered in detail in N. Feather, Electricity and Matter (Edinburgh, 1968), 306–313; and in R. A. Millikan, The Electron (Chicago, 1963), facsimile of 1917, 34–38, 43–47, 51–52, 123–125. esp. in appendices A and B.
The development of the expression for the Townsend discharge and the breakdown equation is discussed in J. A. Crowther, Ions, Electrons, and Ionizing Radiations (London, 1961), 54–56; and is treated technically in A. von Engel, Ionized Gases 2nd ed. (Oxford. 1965), 171–182.
Thaddeus J. Trenn