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The D.C. Electric Motor
The simplest type of d.c. motor consists of a rectangular coil of copper wire placed in a magnetic field.  
   
The ends of the coil are connected to two half cylinders (well ok, slightly less than half), usually made of brass.  
   
These two "half" cylinders are fixed on the axle of the motor and rotate with the coil.  
They form what is called the commutator (which is another word for a switch).  
   
Two brushes (small carbon blocks) are pushed against the two halves of the commutator by springs.  
 
It should be clear that the purpose of the commutator/brushes is to conduct electric current to the moving coil of wire.  
The combination of brass and carbon is chosen because they are both good conductors but also they provide a low friction contact.  
   
   
Consider side a-b of the coil.
Conventional current flows in the sense shown and we can use Fleming's left hand rule to predict that side a-b of the coil will experience a force downwards.
This inevitably means that the force on the other side of the coil will be upwards.
The coil will therefore start to rotate.
The maximum torque on the coil occurs when it is in the position shown here.
 
After one quarter of a rotation, the torque will have reduced to zero because the perpendicular distance between the two forces will be zero (assuming that the field surrounding the coil is at all points parallel to the green line).  
The rotating coil will move under its own momentum and, once past this quarter turn point, the torque will start to increase again.  
Notice that this brushes/commutator arrangement ensures that the torque is always in the same sense.  
In the case shown in the diagram here, the side of the coil on the left, whether it is side a-b or side c-d, will always experience a force downwards because it will always be connected to the left hand brush.  
   
A simple d.c. motor can be constructed from easy-to-obtain bits 'n' pieces... see here.  
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