54 



and 



ELECTRICAL ENGINEERING 



3C = - dynes. 



(84) 



The flux density at the centre of the loop is 



g = 3-C = lines per square centimeter. 



If 7 is expressed in amperes the field intensity at the centre of 

 the loop is 



- 0.2 TT I , 



cM: = dynes . (85) 



(B) At a distance of r cm. from a long straight wire carrying 7 

 absolute units. (Fig. 44.) 



If 3-C is the intensity of the field, the 

 work done in moving a unit magnetic 

 pole around the wire at a distance r cm. 

 from it against the force 3C is 



Unit 

 Pole 



ergs. 



Motion 



The work done is equal to the product 

 of the current and the flux cut by formula 

 and, therefore, 



27ir3C = 47r7, 



and 



27 



FIG. 44. Intensity of the Sfc = dynes; . . . (86) 



magnetic field near a long 



straight wire. tne intensity of the field varies directly 



as the strength of the current and inversely as the distance from 

 the wire. 



The flux density at distance r is 



2 7 



= SK: = lines per square centimeter. 



If 7 is in amperes the field intensity is 



0.27 



dynes, 



(87) 



(88) 



and the flux density is 



027 



lines per square centimeter. . . . (89) 



