Lucent Technologies 5 manual About Telecommunications Telephone Extension Equipment B-2

MERLIN LEGEND Communications System Release 5.0	Issue 1
System Manager’s Guide 555-650-118	June 1997
B About Telecommunications
Telephone Extension Equipment	Page B-2

digital data terminals (for example, personal computers) or advanced videoconferencing equipment. As sophisticated as the equipment has become, many of the basic components and concepts are based on the original telephones.

The first working model of a telephone was demonstrated on March 10, 1876 by Alexander Graham Bell and his assistant, Thomas A. Watson. It consisted of a microphone, called a transmitter, and a small loudspeaker-like device, called a receiver, connected by a pair of wires and a battery.

Early telephones continued to be powered by direct current (dc) supplied by a battery inside the telephone itself until, in 1894, the telephone company used a common battery to power all the telephones connected to an exchange. The telephone company office was called the central office (CO), as it is today, and this was where the battery was located.

The flow of direct current to early telephones was controlled by the receiver (or handset) which hung on a hook that activated electrical contacts. This hook was called a switchhook, a term and concept that is still used today. The status of the switchhook signals the central office about the status of the telephone extension equipment:

■Idle Status. When the handset is sitting on its cradle (on-hook), the switchhook contacts are open (not connected) and no current is drawn from the CO. This signals the CO that the telephone is available to receive calls.

■Busy Status. When the handset is removed from the cradle (off-hook), the switchhook contacts are closed (connected) and current flows. This signals the CO either that the caller is requesting service or that the user is already on a call and is not available for another call.

Likewise, the CO signals the called party by sending current to his or her phone, causing it to ring. When the called party lifts the handset from its cradle, the current flows, indicating to the CO that the party has answered and that it can stop the ringing. Originally, various bells and buzzers were used to signal the called party to pick up the phone. But in 1878, Watson developed a bell-ringer with a hammer attached to an armature which was, in turn, powered by magnetic energy and operated by a hand crank. A form of this ringer is still used in some of today’s telephones.

Identifying which telephone to ring (that is, supplying the identity of the called party) was communicated verbally from the caller to the operator when human operators handled call connections. After automatic switches were in place, telephone numbers were assigned to telephone service subscribers. The caller identified the called party to the switch by dialing the called party’s number. The numbers were originally dialed by using a mechanical device called a rotary dialer with a spring that was wound up by turning it in one direction; on its return to normal position, it created interruptions in the flow of current, generating dial pulses recognized by the switch.