When you pick up the receiver on a telephone, an electrical connection is made to the lines leading to the central office. When you replace the receiver on the cradle the connection is broken or interrupted. This applies to both push-button and rotary dial telephones.
By periodically breaking the connections leading to the central office a number can be dialed. The number of interruptions is equal to the digit dialed, with the exception that ten interruptions corresponds to zero. These pulses may be generated at the rate of ten times per second and there should be a 1/2 second delay between each two digits.
The rotary dial on the telephone is a mechanical device which periodically breaks the connection leading to the central office. When the rotary dial is released, as it travels back to its resting position, it breaks the connection at a rate of ten times per second thus dialing the digit.
Numbers can also be dialed by pushing the cradle switch button at a rate of ten times per second. A solenoid plunger that is mounted to depress and release the cradle switch on the telephone set may be used to dial numbers using the dial pulses technique.