OF TWO FIGURES ON A PLANE. 285 



The correction for the wires at a greater distance is less than one-thousandth 

 per unit of length. 



The total self-induction of the coil is therefore 



n'M + 21 {log -j + 0-11835}, 



where n is the number of windings, and I the length of wire. 



For a circular coil of radius = a, 



M = 4ira (log 8a - log R - 2), 

 where R is the geometrical mean distance of the section of the coil from itself. 



