What device reverses the current?

What is the principle of an electric motor? Name some of the devices in which electric motors are used.

In a d.c. motor, why must the current to the coil be reversed twice during each rotation ?

State what would happen to the direction of rotation of a motor if :

(i) the current were reversed

(ii) the magnetic field were reversed

(iii) both current and magnetic field were reversed simultaneously.

In what ways can a motor be made more powerful?