INTRODUCTION. xlv 



current bridge methods. Inductances calculated from dimensions are in abso- 

 lute electromagnetic units. The ratio of the international to the absolute henry 

 is the same as the ratio of the corresponding ohms. 



Since inductance is measured in terms of capacity and resistance by the bridge 

 method about as simply and as conveniently as by comparison with standard 

 inductances, it is not necessary to maintain standard inductances. They are 

 however of value in magnetic, alternating-current, and absolute electrical meas- 

 urements. A standard inductance is a circuit so wound that when used in a 

 circuit it adds a definite amount of inductance. It must have either such a 

 form or so great an inductance that the mutual inductance of the rest of the 

 circuit upon it may be negligible. It usually is a wire coil wound all in the same 

 direction to make seK-induction a maximum. A standard, the inductance of 

 which may be calculated from its dimensions, should be a single layer coil of 

 very simple geometrical form. Standards of very small inductance, calculable 

 from their dimensions, are of some simple device, such as a pair of parallel wires 

 or a single turn of wire. With such standards great care must be used that the 

 mutual inductance upon them of the leads and other parts of the circuit is negli- 

 gible. Any inductance standard should be separated by long leads from the 

 measuring bridge or other apparatus. It must be wound so that the distributed 

 capacity between its turns is negligible; otherwise the apparent inductance will 

 vary with the frequency. 



POWER AND ENERGY. 



Power and energy, although mechanical and not primarily electrical quanti- 

 ties, are measurable with greater precision by electrical methods than in any 

 other way. The watt and the electric units were so chosen in terms of the c.g.s. 

 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 

 international watt, defined as "the energy expended per second by an unvarying 

 electric current of one international ampere under an electric pressure of one 

 international volt," differs but little from the absolute watt. 



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. 



C.G.S. units are generally used for magnetic quantities. American practice 

 is fairly uniform in names for these units: the c.g.s. unit of magnetomotive force 

 is called the "gilbert," of reluctance, the "oersted," following the provisional 

 definitions of the American Institute of Electrical Engineers (1894). The c.g.s. 

 unit of flux is called the "maxwell" as defined by the 1900 Paris conference. 

 The name "gauss" is used unfortunately both for the unit of induction (A.I.E.E. 

 1894) and for the unit of magnetic field intensity or magnetizing force. "This 

 double usage, recently sanctioned by engineering societies, is based upon the 

 mathematical convenience of defining both induction and magnetizing force 



