INDUCTANCE AND CAPACITY. 357 



Non-inductive circuits. A circuit is said to be non-inductive 

 under given conditions when the inductance of the circuit is neg- 

 ligible, that is, when the electromotive force L x dijdt* is negli- 

 gibly small, as compared with the electromotive force Ri t which 

 overcomes the resistance of the circuit. Thus, a given circuit 

 might be considered to be non-inductive under conditions involving 

 slow .changes .of current, whereas the same circuit could not be 

 considered to be non-inductive under conditions involving very 

 rapid changes of current. When a circuit consists of outgoing 

 and returning wires side by side, its inductance is small and it 

 may usually be considered to be non-inductive. The wires used 

 in resistance boxes are usually arranged non-inductively. This 

 may be done by doubling the wire back on itself, and winding the 

 doubled wire on a spool. In this case the electromotive force 

 between adjacent wires may be great, and the resistance coil may 

 have considerable electrostatic capacity. In order to make a 

 non-inductive resistance coil without this defect, the wire may be 

 wound in one layer on a thin paper cylinder so as to bring the 

 terminals as far apart as possible, 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 simple in shape, and even then the calculation is in 

 most cases quite complicated. The simplest case is given in 

 Art. 7. The inductance of an irregularly shaped coil may be 

 determined by various electrical methods, f 



3. Electromotive force required to cause a current to increase or 

 decrease. To keep a rotating wheel in uniform motion, a torque 

 sufficient to balance the drag of friction must act upon the wheel. 



* See the next article. 



f See Absolute Measurements by A. Gray, Vol. 2, Part 2, pages 438 to 509. 



See also a series of articles in the Bulletin of the United States Bureau of Stand- 

 ards, by E. B. Rosa, Vol. I, page 125, and page 291 ; Vol. 2, page 87, page 161, 

 and page 359 ; Vol. 3, page I. 



