18 



other way. The watt and the electric units were so chosen in terms of the cgs 

 units that the product of the current in amperes by the electromotive force in 

 volts gives the power in watts (for continuous or instantaneous values). The 

 watt is defined as the energy expended per second by an unvarying electric 

 current of 1 ampere under an electric pressure of 1 volt. 



Standards and measurements. — No standard is maintained for power or 

 energy. Measurements are always made in electrical practice in terms of some 

 of the purely electrical quantities represented by standards. 



MAGNETIC UNITS 



Cgs units are generally used for magnetic quantities. American practice is 

 fairly uniform in names for these units: the cgs unit of magnetomotive force 

 is called the gilbert; magnetic intensity, the oersted; magnetic induction, the 

 gauss; magnetic flux, the maxwell, following the definitions of the American 

 Institute of Electrical Engineers (1894). 



Oersted, the cgs emu of magnetic intensity exists at a point where a force 

 of 1 dyne acts upon a unit magnetic pole at that point, i.e., the intensity 1 cm 

 from a unit magnetic pole. 



Maxwell, the cgs emu magnetic flux is the flux through a cm 2 normal to a 

 field at 1 cm from a unit magnetic pole. 



Gauss, the cgs emu of magnetic induction has such a value that if a con- 

 ductor 1 cm long moves through the field at a velocity of 1 cm/sec, length and 

 induction mutually perpendicular, the induced emf is 1 abvolt. 



Gilbert, the cgs emu of magnetomotive force is a field such that it requires 

 1 erg of work to bring a unit magnetic pole to the point. 



A unit frequently used is the ampere-turn. It is a convenient unit since it 

 eliminates 4tt in certain calculations. It is derived from the "ampere turn per 

 cm." The following table shows the relations between a system built on the 

 ampere-turn and the ordinary magnetic units. 11 



11 Dellinger, International system of electric and magnetic units, Nat. Bur. Standards 

 Bull., vol. 13, p. 599, 1916. 



Part 4. — The ordinary and the ampere-turn magnetic units 



Quantity 



Magnetomotive force 5 



Magnetizing force H 



Magnetic flux $ 



Magnetic induction B 



Permeability n 



Reluctance R 



Magnetization intensity J 



Magnetic susceptibility k 



Magnetic pole strength m 



SMITHSONIAN PHYSICAL TABLES 



