MAGNETIC EFFECT OF ELECTRIC CURRENT. 



IOI 



pole by the loop of wire must be equal to the intensity of the 

 field at the pole due to the loop multiplied by the strength of the 

 pole according to equation (16). Consider therefore a magnet 

 pole of strength m placed at the center of the circular loop as 

 shown in Fig. 65. This pole produces a magnetic field of which 

 the intensity at the wire is 

 m/r 2 , and which is every- 

 where at right angles to 

 the wire. Therefore the 

 force with which the wire 

 is pushed sidewise (per- 

 pendicular to the plane of 

 the paper in Fig. 65) is 

 equal to the product of the 

 length of the wire, the in- 

 tensity of the field (m/r 2 ), 

 and the strength of the 

 current / in the wire in abamperes ; but the length of the wire is 

 27rrZ where Z is the number of turns of wire in the loop, so that 

 iirrZ x mfr 2 x / is the force with which the wire is pushed 

 sidewise by the pole m. But, disregarding sign, this is equal to 

 the force mH with which the loop of wire pushes on the pole. 

 Therefore we have 



Fig. 65. 



m 



mH zirrZ x 9 x / 



from which we obtain 



H ' = 



27TZ7 



(32) 



55. Magnetic field in the neighborhood of a long straight electric wire. 



The lines of force of the magnetic field surrounding a long straight electric wire are 

 circles with their planes at right angles to the wire and their centers on the axis of the 

 wire, as explained in Art. 50 and as shown in Fig. 57. To derive an expression for 

 the intensity of this field at a point distant r centimeters from the axis of the wire, 

 proceed as follows : A long straight wire AB carries a current of / abamperes, 

 and a long magnetized steel strip is placed with its north polar edge parallel to AB 

 and at a distance of r centimeters from AB as shown in Fig 66. The magnetic 

 field due to AB has the same value all the way along the polar edge NNNN, as 

 is evident from considerations of symmetry, the wire being indefinitely long. Consider 



