MAGNETISM AND ELECTROMAGNETICS 55 



Unit current would produce a flux density of 2 lines per square 

 centimeter at a distance of 1 cm. from a straight wire and it 

 would produce this flux through a distance of 2 w cm., or it 

 would produce a flux density of 4?r lines per square centimeter 

 through a distance of 1 cm. in air. Thus one absolute unit of 

 current represents a magnetomotive force of 4 ?r gilberts and 

 one ampere represents a magnetomotive force of 0.4 TT gilberts. 



(C) Between two parallel wires A and B, Fig. 45, at a distance 

 of D cm. apart and carrying equal currents I but in opposite 

 directions. 



FIG. 45. Magnetic field between two parallel wires. 



The field intensity or magnetic force at point P distant r cm. 

 from A and D r cm. from B is the resultant of the magnetic forces 

 due to. the currents in A and B. Since these forces act in the 

 same direction at all points between the wires they can be added 

 directly; the field intensity is by formula 



ac = V + ^7 dynes> (90) 



and the flux density at P is 



27 27 



g = 3-c = - - -f- _ lines per square centimeter. (91) 



(D) At any point on the axis of a short coil of radius r cm. 



(Fig. 46.) 



n = number of turns in the coil, 



7 = current in the coil in absolute units. 



Take any point P on the axis at a distance x cm. from the 

 plane of the coil and let 3-C be the field intensity there. If a pole 

 of strength m is placed at P it will be acted on by a force of racJ-C 

 dynes perpendicular to the plane of the coil. The force exerted 



