144 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



resistance of the circuit. Thus, a given circuit might be consid- 

 ered to be non-inductive under conditions involving slow changes 

 of current, whereas the same circuit would not be considered to 

 be non-inductive under conditions involving rapid changes of cur- 

 rent. When a circuit consists simply of out-going and returning 

 wires, side by side, its inductance is so small that it may be in 

 most cases ignored. The wires used in resistance boxes are 

 usually arranged non-inductively. This may be done by doubling 

 the wire back on itself, and winding this doubled wire on a spool ; 

 or the wire may be wound in one layer on a thin paper cylinder, 

 and this cylindrical coil may then be flattened so as to reduce the 

 region (inside) in which the magnetic field is intense. 



Measurement of inductance. The most accurate method for 

 determining the inductance of a coil is by calculation from meas- 

 ured dimensions. This calculation can be carried out only when 

 the coil is very simple in shape, and even then the calculation is 

 in most cases quite complicated.* The simplest case is given in 

 Art. 8 1. The inductance of an irregularly-shaped coil may be 

 determined by various electrical methods, f 



Moment of inertia of a wheel. Analogue of inductance. The 

 kinetic energy of a rotating wheel resides in the various moving 

 particles of a wheel, in the same way that the kinetic energy of a 

 current resides in the various parts of the magnetic field which is 

 due to the current. If the angular velocity &> of the wheel is 

 doubled the linear velocity of every particle of the wheel is 

 doubled, in the same way that the intensity of the magnetic field 

 at every point in the neighborhod of a coil is doubled when the 

 current in a coil is doubled. Therefore the kinetic energy of 

 every particle of a wheel is quadrupled when its angular velocity 

 is doubled, in the same way that the kinetic energy of every por- 

 tion of the magnetic field around a coil is quadrupled when the 



* See a series of articles in the Bulletin of the United States Bureau of Standards, 

 by E. B. Rosa, Vol. I, pages 125 and 291 ; Vol. 2, pages 87, 161 and 359 ; Vol. 3, 

 page i. 



fSee Practical Physics, by Franklin, Crawford and MacNutt, Vol. 2, page 129 ; 

 see also Absolute Measurements, by Andrew Gray, Vol. 2, Part 2, pages 438-509. 



