[11] See Fleming, _Handbook for the Electrical Laboratory_, vol. ii. p. 130. [12] Faraday, _Experimental Researches on Electricity_, vol. i. § 1252. For a very complete set of tables of dielectric constants of solids, liquids and gases see A. Winkelmann, _Handbuch der Physik_, vol. iv. pp. 98-148 (Breslau, 1905); also see Landolt and Börnstein's _Tables of Physical Constants_ (Berlin, 1894). [13] See the following papers by J.A. Fleming and James Dewar on dielectric constants at low temperatures: "On the Dielectric Constant of Liquid Oxygen and Liquid Air," _Proc. Roy. Soc._, 1897, 60, p. 360; "Note on the Dielectric Constant of Ice and Alcohol at very low Temperatures," ib., 1897, 61, p. 2; "On the Dielectric Constants of Pure Ice, Glycerine, Nitrobenzol and Ethylene Dibromide at and above the Temperature of Liquid Air," id. ib. p. 316; "On the Dielectric Constant of Certain Frozen Electrolytes at and above the Temperature of Liquid Air," id. ib. p. 299--this paper describes the cone condenser and methods used; "Further Observations on the Dielectric Constants of Frozen Electrolytes at and above the Temperature of Liquid Air," id. ib. p. 381; "The Dielectric Constants of Certain Organic Bodies at and below the Temperature of Liquid Air," id. ib. p. 358; "On the Dielectric Constants of Metallic Oxides dissolved or suspended in Ice cooled to the Temperature of Liquid Air," id. ib. p. 368. [14] See Faraday, _Experimental Researches_, vol. i. § 1245; R.H.A. Kohlrausch, _Pogg. Ann._, 1854, 91; see also Maxwell, _Electricity and Magnetism_, vol. i. § 327, who shows that a composite or stratified dielectric composed of layers of materials of different dielectric constants and resistivities would exhibit the property of residual charge. [15] Fleming and Ashton, "On a Model which imitates the behaviour of Dielectrics." _Phil. Mag._, 1901 [6], 2, p. 228. [16] The beginner is often puzzled by the constant appearance of the factor 4[pi] in electrical theorems. It arises from the manner in which the unit quantity of electricity is defined. The electric force due to a point-charge q at a distance r is defined to be q/r², and the total flux or induction through the sphere of radius r is therefore 4[pi]q. If, however, the unit point charge were defined to be that which produces a unit of electric flux through a circumscribing spherical surface or the electric force at distance r defined to be ¼[pi]r², many theorems would be enunciated in simpler forms. [17] See Maxwell, _Electricity and Magnetism_, vol. i. § 78b (2nd ed.). [18] Id. ib. vol. i. § 80. Coulomb proved the proportionality of electric surface force to density, but the above numerical relation E = 4[pi][sigma] was first established by Poisson.