Mahogany-cased signalling instrument for use with the Central Telegraph Office (CTO), GPO-College Green street pneumatic tube.
This is the block signalling instrument from the GPO end of the CTO-College Green Branch Office pneumatic tube, used for sending telegram forms between the offices.
Since only a single tube was provided, it was necessary to let the far end know if the tube was occupied. The instruments were also connected by a wire and the clerk would check that the indicator was showing "Tube Clear" before inserting a carrier and a switch was pressed to ring the bell at the far end and change the indicator to "Carrier in Tube" to let staff know that a carrier was on its way.
This tube (originally one of three "street" tubes) was reinstated in November 1929 at the public counter and with the reopening of the CTO in early 1932, was extended there. The tube lasted until the closure of College Green Branch Office in 1952, when St Andrew's Street Branch Office opened.
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