THE CONTINUOUS CURRENT MOTOR 153 



field and to their length; such a force must then act as a 

 tangential force on the periphery of the armature. 



Force Acting on a Conductor in a Magnetic Field. The 

 fundamental formula which we shall use in calculating 

 torque gives the relation between the length of the con- 

 ductor, the strength of the field in which the conductor 

 is lying, the current in the conductor and the force on 

 the conductor. // a conductor I cms. in length lies in a 

 uniform magnetic field of a density of H lines per sq.cm. 

 (direction of conductor being perpendicular to field) and 

 carries a current of I amperes, then the conductor is acted 

 upon by a force which tends to move it in a direction per- 

 pendicular to its length and to the direction of the magnetic 

 field, and the magnitude of this force, in dynes, is given by 

 the equation 



f=HU/lO ....... (25) 



If we wish to express H in lines per sq.in., I in inches, / 

 in amperes, and / in Ibs. we shall have 



2 



(26) 



Illustration of Formula. Suppose that a conductor 

 10 inches long lies in a field of 60000 lines per sq.in. and 

 carries a current of 100 amperes. The force on the con- 

 ductor in Ibs. is evidently 



^ /= 60000 X 100 X 10 X. 885 X 10 ~ 7 = 5.31 Ibs. 

 If we desired the force in dynes we have 



X (10 X 2.54) X = 2,360,000 dynes. 



Direction of Torque the Same for all the Armature Con- 

 ductors. In Fig. 96 is sketched the section of a four-pole 

 motor. The conductors marked (+) are carrying current 



