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Systems of Units

Nature volume 141, pages 11331134 (25 June 1938) | Download Citation



THE February issue of the Journal of the Franklin Institute contains a paper by Prof. W. M. Hall, of the Massachusetts Institute of Technology, on the formation of systems of units, which embodies portions of a 1936 copyright publication on the subject. The conditions for a self-consistent system of units are laid down and for translational kinematics length and time taken as fundamental. Translational kinetics then gives mass and rotational angle. Thermodynamics introduces temperature, light visibility and sound audibility. Electrodynamics admits of two possibilities: permittivity or permeability may be taken as fundamental and give the electrostatic or electromagnetic systems respectively. The questions of the sizes of the units to be chosen for convenience and of rationalizing or subrationalizing the units in which the coefficient 4iu appears are discussed. Tables are given in which the units of the following systems are compared. The electromagnet with centimetre, gram, second and permeability of vacuum fundamental, the electrostatic with permittivity replacing permeability, and rationalized and subrationalized forms of each, the Gaussian mixed system with some quantities expressed in electromagnetic others in electrostatic units, the rationalized form of this, introduced by Heaviside and Lorentz, the Maxwell quadrant (109 cm.), 10-11 gram, second and permeability of vacuum, the Bennett and others, centimetre, 107 gram, and a unit of permittivity of vacuum = l/(8·989 x 1011), the Giorgi, metre, kilogram, second and 107 times the permeability of vacuum. The last three systems all have the ampere, volt and watt as units of current, potential and power respectively.

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