BEST FORM OF THE CHANNEL. 



wires and bring them to another point. We shall assume that the 

 magnet is infinitely small, and placed at the centre of the coil. 



That being admitted, let us consider the meridional section ; let 

 P (Fig. 136) be a point of the bounding curves, r the radius of the 

 winding, and a the distance of its plane from the surface. As the 

 specific action of the winding at the point O should be the same, 



Fig. 136. 



whatever be the position of the point P on the curve, this will be 

 defined by equation 



const = . 



If u is the radius vector OP of the point P, and the angle of 

 this radius with the axis, the equation becomes 



(23) 



= ^ sn 



The curve represented by equation (23) has the form of a circle 

 flattened along the vertical diameter. The tangent is vertical at the 

 point of intersection with the straight line, the angular coefficient of 



which is equal to. Figure 136 gives the successive curves for 



