released, the dial un-winds at a constant speed. The rotating
motion causes a toothed wheel (the pulse wheel) to repeatedly
open and close an electrical contact wired directly into
the telephone line.
making and breaking of the circuit is detected at the exchange
as a series of pulses corresponding to the number dialled
(number 0 [zero] creates 10 pulses).
Telephone keypad (red zone shows the
normally unavailable extra buttons)
conventional dial of the telephone was replaced by a 12
button keypad. When a key is pressed the tone of the column
(High Tone) and the tone of the row (Low Tone) are simultaneously
generated (eg, pressing the '5' button generates the tones
of 770Hz and 1336H). This dual tone is sent down the telephone
line to the exchange.
tones were chosen to avoid harmonics (no tone/frequency
is a multiple of another, the difference between any two
tones doesn't equal any of the frequencies, and combining
two tones does not equal any of the other tones). The frequency,
amplitude and time duration of each dual-tone is defined
in various international technical specifications (eg CCITT).
A, B, C and D buttons are not available on ordinary phones.
They were originally used by the US military, and are now
used for signalling on the telephone circuits between exchanges.
"#" key is known by several different names
including Hash, Pound, Crosshatch, etc.