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 