Relations between Electrical Measurements. 459 



The measure of electromotive force used by W. Weber, and 

 derived by him (independently of the principle of conservation 

 of energy) from the motion of a conductor in a magnetic field, 

 is the same as that at which we have arrived ; for, from equa- 

 tion (15), we find that the unit electromotive force will be pro- 

 duced by motion in a magnetic field when one line of force is 

 added (or subtracted) per unit of time; and this will occur when 

 in a field of unit intensity a straight bar of unit length, forming 

 part of a circuit otherwise at rest, is moved with unit velocity 

 perpendicularly to the lines of force and to its own direction. 



To W.Weber, whose numerical determinations of electrical mag- 

 nitudes are the starting-point of exact science in electricity, we 

 owe this, the first definition of the unit of electromotive force ; 

 but to Professor Helmholtz* and to Professor W. Thomson j*, 

 working independently of each other, we owe the proof of the 

 necessary existence of magneto-electric induction, and the deter- 

 mination of electromotive force on strictly mechanical principles. 



32. On Material Standards for the Measurement of Electrical 

 Magnitudes. — The comparison between two different electrical 

 magnitudes of the same nature, e. g. between two currents or 

 between two resistances, is in all cases much simpler than the 

 direct measurement of these magnitudes in terms of time, mass, 

 and space, as described in the foregoing pages. Much labour is 

 therefore saved by the use of standards of each magnitude ; and 

 the construction and diffusion of those standards form part of 

 the duties of the Committee. 



Electric currents are most simply compared by " electro-dyna- 

 mometers" (20) — instruments which, unlike galvanometers, are 

 practically independent of the intensity of the earth's magnetism. 

 When an instrument of this kind has been constructed, with 

 which the values of the currents corresponding to each deflection 

 have been measured (19), (20), other instruments may easily be 

 so compared with this standard that the relative value of the 

 deflections produced by equal currents on the standard and the 

 copies shall be known. Hence the absolute value of the current 

 indicated by each deflection of each copy will be known in abso- 

 lute measure. In other words, in order to obtain the electro- 

 magnetic measure of a current in the system described, each 

 observer in possession of an electro-dynamometer which has been 

 compared with the standard instrument will simply multiply by 

 a constant number the deflection produced by the current on his 

 instrument (or the tangent or sine of the deflection, according 

 to the particular construction of the instrument). 



* Paper read before the Physical Society of Berlin, 1847 {vide Taylor's 

 Scientific Memoirs, part 2. Feb. 1853, p. 114). 

 t Reports of the British Association, 1848; Phil. Mag. Dec. 1851. 



2H 2 



