The commutator is a ring with a cut, in contact with fixed devices on the shaft, called brushes, where the chain will be supplied. Every half turn the sides of the loop (light and dark) will always be in contact with different brushes, then inverting the electric current in the loop.
In the case of an alternating current (AC) motor, the commutator is not necessary because the separate aneon.
The electric metering instruments have a galvanometer, in which the same phenomenon of the DC motor is observed. When the electric current travels through the coil, the magnetic force moves the pointer. Thus, a spring acting on force contrary to the magnetic force causes the pointer to stop, showing the value of the magnitude to be measured. The larger this magnitude, the greater the deflection.
The operating principle of a relay and by means of the action of the magnetic force that appears in the attraction between the movable and the fixed part and when energizing the coil.
The mobile core feels the attraction that moves the contacts, changing position. Since the movable contacts are fixed in the mobile core part, a contact that was previously closed (normally closed – NF) is opened, and the other one that was open (normally open – NA) is closed.