right hand is pointed in the direction of the magnetic field  and the second finger in the direction of current flow, the  thumb indicates the direction the current carrying wire moves.  The amount of torque developed in a coil depends upon several  factors: the strength of the magnetic field, the number of turns  in the coil, and the position of the coil in the field. Magnets are  made of special steel that produces a strong field. Since there  is torque acting on each turn, the greater the number of turns  on the coil, the greater the torque. In a coil carrying a steady  current located in a uniform magnetic field, the torque varies  at successive positions of rotation.  [Figure 12-295]  When the  plane of the coil is parallel to the lines of force, the torque is  zero. When its plane cuts the lines of force at right angles, the  torque is 100 percent. At intermediate positions, the torque  ranges between 0 and 100 percent.  A coil of wire through which the current flows rotates when  placed in a magnetic field. This is the technical basis governing  the construction of a DC motor.  [Figure 12-296]  However,  if the connecting wires from the battery were permanently  fastened to the terminals of the coil and there was a flow of  current, the coil would rotate only until it lined itself up with  the magnetic field. Then, it would stop, because the torque  at that point would be 0.  A motor, of course, must continue rotating. It is therefore  necessary to design a device that reverses the current in the  coil just at the time the coil becomes parallel to the lines of  force. This creates torque again and causes the coil to rotate.  If the current reversing device is set up to reverse the current  each time the coil is about to stop, the coil can be made to  continue rotating as long as desired.  One method of doing this is to connect the circuit so that, as  the coil rotates, each contact slides off the terminal to which it  connects and slides onto the terminal of opposite polarity. In  other words, the coil contacts switch terminals continuously  as the coil rotates, preserving the torque and keeping the  coil rotating. In  Figure 12-296 , the coil terminal segments  S  N  45°  71% 100%  90°  0°  Zero torque  S N  A  B  Brush  Torque  S N  A  B  No torque  S N  B  A  Torque  S N  B  A  No torque  A  B  C  D  Figure 12-295.  Torque on a coil at various angles of rotation.  Figure 12-296.  Basic DC motor operation.  12-141  Basic DC Motor 