This key and lamp unit allows the signalman to speak to any one of ten incoming 2-wire telephone circuits. This type of equipment in varnished wooden cabinets was already old-fashioned when I started to learn about telecommunications in the 1960s. The two volumes of 'Telephony' by Atkinson were the telecommunications 'Bible' and gave an insight into the principles used.
On the left of the box the operator's handset can be seen, hanging from the 'Gravity Switch'. Originally, this would have been a heavy, bakelite 'Hand Micro-Telephone' ('HMT') but it has been replaced by a more modern, and much lighter, '700' series model. The handset cord terminates in a Plug which connects with a Jack mounted on the cabinet.
The horizontal row of coloured knobs in the middle of the front of the cabinet are 3-position Lever Keys, one for each outgoing line. They're often called 'Kellog Keys' after the original manufacturer. Lifting the key upwards sends ringing current to the associated line, so as to ring the bell of the remote telephone. The key is biased so that it returns to the normal position as soon as the operator lets go. This is called 'Non-locking' action. Pushing the key downwards connects the operator's handset to the associated line, so that two-way speech is possible. In this direction, the key will stay down when the operator lets go, leaving the operator with one hand free during the conversation - useful if written notes are required. This is called 'Locking' action. At the end of the call, the operator flicks the lever key back to the central position, disconnecting the telephone he was speaking to.
Below the row of lever keys is a metal 'Lamp Strip'. This mounts ten lampholders accepting plug-in filamentary lamps, usually the British Post Office No. 2 'wedge' lamp. An opal white glass lens fitted in a clip-in brass mounting is fitted in front of each lamp. When any of the remote telephones initiates a call, the appropriate lamp glows white and a buzzer (common to all lines) sounds until the operator pushes the associated key down to the 'SPEAK' position. This action silences the buzzer and allows a conversation to take place.