Field of an Electrical Current. 



219 



In fig. 3 let EHB represent half the cross-section of the 

 wire, its centre being D, and let the centre, (see fig. 1), be 

 in the production of the line DE at a distance 10 millim. 

 above D, while DE = 1. Let a series of circles be described 



ronnd D with radii Df Dg, Dh, . . . equal to 1*1, 1% 1*3, 

 1*4, 1*5 millim., and suppose that we trace the line of force 

 which touches the wire at E. If we calculate the angle <f> 

 which defines the point in which this line cuts the circle of 

 radius Df we find, taking only terms of the first order, 

 e=ED/, or 



e = 63° 21'. 



This gives the point p'; but taking the terms of the second 

 order the angle becomes 62° 37', which gives the true point, 



p, by means of the angle ED^> 



Q2 



Similarly the point, </, in 



