Roever — Geometrical Constructions of Lines of Force. 22 1 



w = 4 and — m' = — 1, respectively; and Fig. 9 shows a few 

 of the lines of force proceeding from a system consisting of 



two electrified points A and A' having charges wi = 4 and 

 m' = I respectively. 



If m = m', numerically, equation (24) becomes 

 (1 — cos co) — (1 — GOS eo') = 1 — cos a 



or 



cos 0)' — cos CO = 1 — COS a. 



.(28). 



This is the equation of a closed curve passing through A 

 and A. A line of force is only an arc of this curve. 



If 0) = G)' = ^, then, from equation (24), 



971 



versin d = 1 — cos d = > (1 — cos «). . ( 29). 



This determines the direction of the asymptote to a line of 

 force. 



