120 COEFFICIENTS OF INDUCTION. 



i oth. The geometrical mean distance of all points of a corona 

 of radii a and a l will be deduced from the geometrical mean distance 

 from a point to the corona, which gives 



(d'-a^y a^ 4 cr - a\ 

 From this we get, for a circle of radius a, 



or 



and for a circumference of radius a, 



759. SELF-INDUCTION OF Two PARALLEL WIRES. We have seen 

 (566) that the properties of an electromagnetic field are defined by 

 the electromotive force J at each point. The components of the 

 induction are expressed (567) as a function of the components of 

 the electromotive force, and the component of the current by the 

 aid of those of induction. 



If the field only contains parallel currents in cylindrical con- 

 ductors, and we take the 2-axis parallel to the currents, the electro- 

 motive force is also parallel to the 2-axis from symmetry. All the 

 phenomena depend -then only on the component H. 



Let us consider a linear current of intensity I at the origin of 

 co-ordinates. At the distance r from the axis, the force F only 

 depends on r ; taking the point on the ^-axis, we have 



Changing the direction of the y, to have the ordinary representation 

 in a plane, equation (2) of (567) gives 



an an 21 



