A Rotary Or Mechanical Dial is an impulse sending device. When the subscriber lifts his handset, the D.C. loop is completed and steady current flows through the line provided by the exchange. The impulsing cam of the dial breaks the circuit as many times as the number dialed, thus producing pulses of current and sends it over the subscriber's line to the exchange.
The Rotary Or Mechanical Dial consists of finger plate with ten holes in it. These holes are equally spaced around 2/3rd of the outer ring of the finger plate. The numbers 1 to 0 are written on a number ring below the finger plate. These numbers can be seen through the ten holes of the finger plate. There is a finger stop adjacent to the digit "0". The rotation of the fingerplate is clockwise against the tension of a spring that restores it to its normal position, after dialing a digit. During the anti clockwise or reverse movement of-the finger plate, the speed of the plate is kept constant with the help of a "Governor". The governor is a mechanical device and consists of a number of weights on the spring. It moves along with the dial through gear assembly.
- when dialing, due to the interruption of current clicks are produced and are acceptable. The off-normal contacts are used to bypass the speech circuit wh3 current from the exchange flows through them and.
- when the contacts close the impedance of the line reduces, hence the current increases. The constructional detail of the rotary or mechanical dial is shown in the figure