Page:EB1911 - Volume 27.djvu/768

TABLE II.—Observed Values of v in Centimetres per Second

Date.	Name.	Reference.	Electric Quantity Measured.	v in Centimetres per Second.
1856	W. Weber and R. Kohlrausch	Electrodynamische Massbestimmungen  and Pogg. Ann. xcix., August 10, 1856	Quantity	3·107 × 1010
1867 1868	${\Big \}}$Lord Kelvin and W. F. King	Report of British Assoc., 1869. p. 434;  and Reports on Electrical Standards,  F. Jenkin, p. 186	Potential	2·81  × 1010
1868	J. Clerk Maxwell	Phil. Trans. Roy. Soc., 1868, p. 643	Potential	2·84  × 1010
1872	Lord Kelvin and Dugald M‘Kichan	Phil. Trans. Roy. Soc., 1873, p. 409	Potential	2·89  × 1010
1878	W. E. Ayrton and J. Perry	Journ. Soc. Tel. Eng. vol. viii. p. 126	Capacity	2·94  × 1010
1880	Lord Kelvin and Shida	Phil. Mag., 1880, vol. 10, x. p. 431	Potential	2·995 × 1010
1881	A. G. Stoletow	Soc. Franc. de Phys., 1881	Capacity	2·99  × 1010
1882	F. Exner	Wien. Ber., 1882	Potential	2·92  × 1010
1883	Sir J. J. Thomson	Phil. Trans. Roy. Soc., 1883, p. 707	Capacity	2·963 × 1010
1884	I. Klemencic	Journ. Soc. Tel. Eng., 1887, p. 162	Capacity	3·019 × 1010
1888	F. Himstedt	Electrician, March 23, 1888, vol. xx. p. 530	Capacity	3·007 × 1010
1888	Lord Kelvin, Ayrton and Perry	British Association, Bath; and Electrician,  Sept. 28, 1888	Potential	2·92  × 1010
1888	H. Fison	Electrician, vol. xxi. p. 215; and Proc. Phys.   Soc. Lond., June 9, 1888	Capacity	2·965 × 1010
1889	Lord Kelvin	Proc. Roy. Inst., 1889	Potential	3·004 × 1010
1889	H. A. Rowland	Phil. Mag. 1889	Quantity	2·981 × 1010
1889	E. B. Rosa	Phil. Mag., 1889	Capacity	3·000 × 1010
1890	Sir J. J. Thomson and G. F. C. Searle	Phil. Trans., 1890	Capacity	2·995 × 1010
1891	M. E. Maltby	Wied. Ann. 1897	Alternating  Currents	3·015 × 1010

In connexion with the numerical values in the above definitions much work has been done. The electrochemical equivalent of silver or the weight in grammes deposited per second by 1 C.G.S. electromagnetic unit of current has been the subject of much research. The following determinations of it have been given by various observers:—

Name.	Value.	Reference.
E. E. N. Mascart	0·011156	Journ. de physique, 1884, (2), 3, 283.
F. and W. Kohlrausch	0·011183	Wied. Ann., 1886, 27, 1.
Lord Rayleigh and Mrs Sedgwick	0·011179	Phil. Trans. Roy. Soc., 1884, 2, 411.
J. S. H. Pellat and A. Potier	0·011192	Journ. de Phys., 1890, (2), 9, 381.
Karl Kahle	0·011183	Wied, Ann., 1899, 67, 1.
G. W. Patterson and K. E. Guthe	0·011192	Physical Review, 1898, 7, 251.
J. S. H. Pellat and S. A. Leduc	0·011195	Comptes rendus, 1903, 136, 1649.

Although some observers have urged that the 0·01119 is nearer to the true value than 0·01118, the preponderance of the evidence seems in favour of this latter number and hence the value per ampere-second is taken as 0·0011800 gramme. The exact value of the electromotive force of a Clark cell has also been the subject of much research. Two forms of cell are in use, the simple tubular form and the H-form introduced by Lord Rayleigh. The Berlin Reichsanstalt has issued a specification for a particular H-form of Clark cell, and its E.M.F. at 15° C. is taken as 1·4328 international volts. The E.M.F. of the cell set up in accordance with the British Board of Trade specification is taken as 1·434 international volts at 15° C. The detailed specifications are given in Fleming’s Handbook for the Electrical Laboratory and Testing Room (1901), vol. i. chap. 1; in the same book will be found copious references to the scientific literature of the Clark cell. One objection to the Clark cell as a concrete standard of electromotive force is its variation with temperature and with slight impurities in the mercurous sulphate used in its construction. The Clark cell is a voltaic cell made with mercury, mercurous sulphate, zinc sulphate, and zinc as elements, and its E.M.F. decreases 0·08% per degree Centigrade with rise of temperature. In 1891 Mr Weston proposed to employ cadmium and cadmium sulphate in place of zinc and zinc sulphate and found that the temperature coefficient for the cadmium cell might be made as low as 0·004 % per degree Centigrade. Its E.M.F. is, however, 1·0184 international volts at 20° C. For details of construction and the literature of the subject see Fleming’s Handbook for the Electrical Laboratory, vol. i. chap. 1.

In the British Board of Trade laboratory the ampere and the volt are not recovered by immediate reference to the electrochemical equivalent of silver or the Clark cell, but by means of instruments called a standard ampere balance and a standard 100-volt electrostatic voltmeter. In the standard ampere balance the current is determined by weighing the attraction between two coils traversed by the current, and the ampere is defined to be the current which causes a certain attraction between the coils of this standard form of ampere balance. The form of ampere balance in use at the British Board of Trade electrical standards office is described in Fleming’s Handbook for the Electrical Laboratory, vol. i., and that constructed for the British National Physical Laboratory in the report of the Committee on Electrical Standards (Brit. Assoc. Rep., 1905). This latter instrument will recover the ampere within one-thousandth part. For a further description of it and for full discussion of the present position of knowledge respecting the values of the international practical units the reader is referred to a paper by Dr F. A. Wolff read before the International Electrical Congress at St Louis Exhibition, U.S.A., in 1904, and the subsequent dispassion (see Journ. Inst. Elec. Eng. Lond., 1904–5, 34, 190, and 35, 3.

The construction of the international ohm or practical unit of resistance involves a knowledge of the specific resistance of mercury. Numerous determinations of this constant have been made. The results are expressed either in terms of the length in cm. of the column of pure mercury of 1 sq. mm. in section which at 0° C. has a resistance of 10⁹ C.G.S. electromagnetic units, or else in terms of the weight of mercury in grammes for a column of constant cross sectional area and length of 100·3 cm. The latter method was adopted at the British Association Meeting at Edinburgh in 1892, but there is some uncertainty as to the value of the density of mercury at 0° C. which was then adopted. Hence it was proposed by Professor J. Viriamu Jones that the re determination of the ohm should be made when required by means of the Lorentz method (see J. V. Jones, “The Absolute Measurement of Electrical Resistance,” Proc. Roy. Inst. vol. 14, part iii. p. 601). For the length of the mercury column defining the ohm as above, Lord Rayleigh in 1882 found the value 106·27 cm., and R. T. Glazebrook in the same year the value 106·28 cm. by a different method, while another determination by Lord Rayleigh and Mrs Sedgwick in 1883 gave 106·22 cm. Viriamu Jones in 1891 gave the value 106·30 cm., and one by W. E. Ayrton in 1897 by the same method obtained the value 106·27 to 106·28 cm. Hence the specific resistance of mercury cannot be said to be known to 1 part in 10,000, and the absolute value of the ohm in centimetres per second is uncertain to at least that amount. (See also J. Viriamu Jones, “On a Determination of the International Ohm in Absolute Measure,” Brit. Assoc. Report, 1894.)

The above-described practical system based on the C.G.S. double system of theoretical units labours under several very great disadvantages. The practical system is derived from and connected with an abnormally large unit of length (the earth quadrant) and an absurdly small unit of mass. Also in consequence of the manner in Rational system of electrical units. which the unit electric quantity and magnetic pole strength are defined, a coefficient, 4π, makes its appearance in many practical equations. For example, on the present system the magnetic force H in the interior of a long spiral wire of N turns per centimetre of length when a current of A amperes circulates in the wire is 4π AN/10. Again, the electric displacement or induction D through a unit of area is connected with the electric force E and the dielectric constant K by the equation- D=KE/4π. In numerous electric and magnetic equations the constant 4π makes its appearance where it is apparently meaningless. A system of units in which this constant is put into its right place by appropriate definitions is called a rational system of electric units. Several physicists have proposed such systems. Amongst others that of Professor G. Giorgi especially deserves mention.Giorgi’s system of electrical units. We have seen that in expressing the dimensions of electric and magnetic qualities we cannot do so simply by reference to the units of length, mass and time, but must introduce a fourth fundamental quantity. This we may take to be the dielectric constant of the ether or its magnetic permeability, and thus we obtain two systems of