Rotary Or Mechanical Dial

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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.

The Rotary Or Mechanical Dial also consists of an impulsing contact. This contact is normally closed. When the dial operates, a cam opens and closes this contact, thus producing pulses of current on the line. The number of pulses produced corresponds to the number of times the contact closes. The adjustment of the dial is such that it produced 10 pulses per second, whereas ± 10% variation is acceptable. The number of pulses generated by the dial corresponds to the digit dialed. For example,

By dialing digit 1, one pulse will be produced.
By dialing digit 2, two pulses will be produced.
By dialing digit 3, three pulses will be produced.
By dialing digit 0, ten pulses will be produced.

The dial also consists of off-normal contacts. These contacts are normally open or off. These contacts remain close, when the dial move forward and then come back to rest position. There are two advantages of these contacts,

  1. 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.
  2. 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

Rotary Or Mechanical Dial


Constructional details of Rotary Or Mechanical Dial


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