The Convention of Equidimensional Electric and Magnetic Units

VOL. 17, 1931

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147

involves s2 is composed of the smallest subgroup of G which gives rise to a quotient group which is either dihedral or generalized dihedral. Hence it follows that the concepts of inverse commutator subgroup and special inverse commutator subgroup enable us to unify a number of fundamental theorems of groups of finite order. This unification is the main object of the present note. While the identity automorphism gives rise only to the identity commutator it gives rise to the squares of all the operators of the group as inverse commutators. 1 G. A. Miller, Trans. Am. Math. Soc., 10, 472 (1909).

THE CONVENTION OF EQUIDIMENSIONAL ELECTRIC AND MAGNETIC UNITS By ARTHUR E. KENNELLY HARVARD UNIVERSITY AND MASS. INST. OF TECHNOLOGY

Communicated February 4, 1931

It is the object of this paper to present a set of electromagnetic unit dimensions, common to both electric and magnetic systems, based on the conventional hypothesis that the dimensions of permittivity Ko and permeability guo for free space are the same. It was announced by Maxwell in 1881 in his Treatise,' Vol. 2, Chap. X, that "every electromagnetic quantity may be defined with reference to the fundamental units of Length, Mass, and Time." He formulated the "dimensions" of each quantity, such as Resistance, Current, Capacitance, etc., in terms of the fundamental quantities of dynamics-L, M, and T. Thus with any velocity V, defined as a ratio LIT, the dimensions of velocity would be L1, MO, T-', or giving only the exponents, as (1, 0, - 1). Similarly, the ordinary dynamic formulas of energy W, and of power P, being respectively MV2/2 and WIT, their exponential dimensional formulas would be (2, 1, -2) and (2, 1, -3). Maxwell also showed that there were always two different dimensional formulas for each electromagnetic quantity: namely, one in the electric (electrostatic) system, and one in the magnetic (electromagnetic) system. The electric units were derived from the force of repulsion between like electric charges across a known distance, assuming that the permittivity Ko of free space is the numeric unity. The magnetic units were, however, derived from the force of repulsion between like magnetic poles across a known distance, assuming that the permeability go of free space is the numeric unity. The accompanying table 1 is based on Maxwell's list of dimensions for

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ENGINEERING: A. E. KENNELLY

PROC. N. A. S.

the electric and magnetic units. Column II names the quantities considered, III gives the customary distinguishing symbol, IV, V and VI the dimensions of the quantity in the magnetic system; while VIII, IX and X give them in the electrostatic system. To take a simple example, No. 9 in the list, Resistance R, has dimensions L1, MO, T-1, or those of velocity V = L/T in the magnetic system; but has L-1, MO, TP, or those of inverse velocity V-1 = T/L, or slowness, in the electric 4 system. Maxwell's findings in these matters, as repeated in Columns IV, V, VI, and VIII, IX, X, have never been disputed, taking into account the postulates he assumed. On the other hand, they have never been regarded as self-explanatory, except perhaps, Electric Capacitance, No. 6, and Magnetic Inductance, No. 18, each of which appears with the dimensions of a simple Length, and for each of which a physical realization has been suggested by several writers. It wa's pointed out by Rucker2 in 1889, that Maxwell's list of unit dimensions was necessarily incomplete; because the constants of free space (permittivity KO and permeability iuo) are involved in the action of the observed forces between electric charges or magnetic poles. These space constants are suppressed in Maxwell's lists, and yet they must be assumed to have dimensions of some sort. Indeed, Maxwell showed in his Treatise,' Chap. XX, that

VZO-I.&O= vI

(1)

where v is the velocity of electromagnetic propagation, or approximately 300,000 km. per second. Rucker introduced these unresolved space constants, with their appropriate exponents, into Maxwell's dimensional formulas, as indicated in Columns VII and XI of Table 1. Thus Resistance R (No. 9) has revised Maxwell-Rucker dimensions of DA = LI, MO, T-1, ,U1 in the magnetic system and DK = L-1, MO, T, K-1, in the electric system. These Maxwell-Rucker dimensions, as repeated in Table 1, under D,. and DA, have been recognized since 1889, and are quoted in various text-books and handbooksA'456. From Maxwell's equation (1), we have: Koo =go

(2)

or the product of the two space constants is the inverse square of the propagation velocity. This equation admits, of course, of an infinite number of physical and dimensional solutions for KO and uo, separately. In the electric system of units, KO is taken as unity and Ao as 1/v2; whereas in the magnetic system, uo is taken as unity and KO as 1/v2.

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VOL. 17, 1931

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Although several different suggestions have been made at different times, for the solution of Maxwell's equation (1), based on physical possibilities; yet, setting physical considerations aside, the simplest solution is evidently Ko = guo = 1/v, or L-1, MO, T'. That suggestion has been made;7 but so far as the writer is aware, without pointing out the results of this convention upon the series of electric and magnetic unit dimensions, such as those in table 1. If we divide D, by DK, as in Columns XII, XIII and XIV, we obtain a series which shows that not only are these ratios all dimensionless, since (#K) = L-2T2; but also that on the equidimensional convention, D,, = DK, or the two series D. and D. of electric and magnetic dimensions, each of four elements (L, M, T and KO or L, M, T and ,uo) coalesce into a single three-element series (L, M, T) of the Maxwellian type. Thus the equidimensional convention for Ko and ,uo leads to an equidimensional series for all the existing units. Moreover, it seems that the equidimensional solution of the Maxwell equation (1) is the only one which yields equidimensional electric and magnetic series in table 1. Table 2 shows, in somewhat rearranged form, the single series of dimensions for both electric and magnetic units belonging to any of the recognized systems (C. G. S., and Practical) as obtained from table 1, when ,Ao and KO are replaced by L-'T', the dimensions for the propagation slowness of approximately 1/(3 X 108) seconds per meter. Columns I and XV contain the successive entry numbers, with their dimensions in VII, VIII, and IX. Columns II and XIV give characteristic or descriptive dimensions. Columns III and V give the electrokinetic and electrostatic quantities defined. Their ordinary symbols appear in IV and VI. Columns XI and XIII likewise give the magnetostatic and magnetokinetic quantities. The former pertain to the ordinary steady magnetic circuit, and the latter to the alternating magnetic circuit. (1) El. Energy EQ, and Mag. Energy Fb have (2, 1,-2), in common with dynamic energy W.

(2) El. Power E2G, EI, or F2R, and Mag. Power F dt have (2, 1, -3), dt in common with dynamic power P = WIT. (3) El. Permittivity K and Mag. Permeability ,u have, by assumption, the slowness dimensions (-1, 0, 1). (4) Elastivity 1/1 and Reluctivity v = l/,u have, by the same assumption, the velocity dimensions (1, 0, -1). (5) Both Electric and Magnetic Motive Forces, E, F, and- potentials

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ENGINEERING: A. E. KENNELLY

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(7) Capacitance C, Permeance P and Inductance L all have the dimensions (0, 0, 1) of time (seconds), The same is trute of the wellknown circuit time-constants CR and L/R. (8) El. Quantity IT, Charge Q and static flux be together with Mag. Quantity m, Pole strength m, and Mag. Flux 4) have (1, 1/2, - 1/2) or root time-energy VTW, or time root power TX/P. (9) El. and Mag. Surface densities and flux densities have (-1, 1/2, - 1/2) or root time-energy per unit area, which conforms with B2/(2 Po) the well-known expression of El. or Mag. Energy per unit volume. (10) The El. and Mag. Forces He and H, or gradients of potential, have (0, 1/2, - 3/2) or N/P/L. Their product He H is thus PIS, which conforms with Poynting's Theorem. (11) The Electrokinetic Resistances and Conductances have all zero dimensions, or are mere numerical coefficients of power delivery. (12) El. Resistivity p has the dimensions of length. (13) El. Conductivity y has the dimensions of inverse length. The question of the utility of the equidimensional convention may therefore be answered as follows: (a) It simplifies and unifies the Table of Dimensions, which can then be readily memorized. (b) Its dimensional expressions are mutually consistent and are closely associated with those of energy and power. (c) It colligates and unites quantities in the electric and magnetic circuits of corresponding energy relations. In comparing tables 1 and 2, it may be observed that the equidimensional series of table 2 gives, for each quantity, the arithmetical mean of the Maxwellian El. and Mag. dimensions in table 1. In other words, the equidimensional series is also the mean-dimension series. The question as to what relations may exist between the equidimensional electromagnetic quantities and those in the actual physical universe, is much larger and more serious than any thus far here discussed. It is not easy to explain why the space constants elastivity and reluctivity should be velocities, capacitance and inductance times, and resistance only a coefficient. It may be that there remain yet other suppressed dimensions here ignored; or that the nature of electromagnetic phenomena may not admit of adequate representation in purely dynamic terms. For the present, we may be content to regard the equidimensional convention as a device for simplifying and condensing existing tables of electromagnetic dimensions. BIBLIOGRAPHY

1J. Clerk Maxwell, A Treatise 1881.

on

Electricity ani Magnetism, 2nd Ed., Oxford,

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2 A. W. Rucker, "On the Suppressed Dimensions of Physical Quantities," Phil. Mag., 1889, pp. 104-114. 3 Carl Hering, Conversion Tables, New York, 1904. 4E. Bennett, "A Digest of the Relations between the Electrical Units and the Laws Underlying the Units," Univ. of Wisconsin Bull., 1917. 6 Smithsonian Physical Tables, Sixth Edition, Washington, 1916. B Bureau of Standards, "Electric Units and Standards," Circular 60, 1920. 7 A. E. Kennelly, "Magnetic Circuit Units," Trans. Am. Inst. El. Engrs., Jan., 1930, 49, No. 2, Apr., 1930, pp. 486-510. Discussion by Gokhale, p. 503, Tables II and III.

ON THERMOD YNAMIC EQUILIBRI UM IN A STA TIC EINSTEIN UNIVERSE By RICHARD C. TOLMAN NORMAN BRIDGE LABORATORY, CALIFORNIA INSTITUTE OF TECHNOLOGY

Communicated February 10, 1931

§ 1. Introduction.-It has now become evident that the transformation of matter into radiation taking place throughout the universe and the red-shift observed in the light from the extra-galactic nebulae appear to imply a non-static quality in the universe which can be treated with some success with the help of a non-static cosmological line element.' If the quantity which gives the dependence of this non-static line element on the time is set equal to a constant, it is found that the line element then becomes the same as Einstein's original line element for a static universe. Hence the Einstein static universe may be regarded as a special case of the more general non-static universe, and we must continue to be interested in the properties of the Einstein universe not only because it is a limiting case of the more general model for the universe, but also because it represents a situation which might arise in the course of the evolution of the actual universe. The present article will deal with the thermodynamics of the Einstein universe, and in particular will treat the conditions for thermodynamic equilibrium between matter and radiation in such a universe assuming the possibility of their transformation into each other. Treatments of the general problem of the equilibrium between matter and radiation have already been given for the case of a perfect monatomic gas interacting with black body radiation both in the absence and presence of gravitational fields. In the absence of any appreciable gravitational field, it was shown by the work of Stern2 and myself3 that the number of monatomic molecules of mass m present in unit volume at equilibrium at temperature T would be given by a formula of the form mcS

N

=

bT31/2 e

kT

(1)