ATDTDA (2): pneumatic house-tube (39.34)

Wed Feb 7 10:30:53 CST 2007

When the application, having been sent off to some invisible desk up the other end of a pneumatic house-tube, at length came thumping back handstamped "Approved" [...] (p. 39)

[...] Two types of system were eventually adopted, house and street tubes. House tubes provided for the transmission of messages between different part of the same building, street tubes provided for transmission of telegram forms from branch offices to the head office (from whence they were telegraphed). By the 1930s, 67 branch offices were connected to the head office by a series of radial tubes. Most tubes conveyed messages in one direction only, some in both directions. At its peak, the London network made use of 57 miles of pipeline [5]. The system was used because it allowed quicker handling of messages than would be the case if messages were telegraphed from local offices to head office. There was also no error in translation.

Street tube pipes were made of lead, laid 30 cm under the street within iron ducts, which provided 'mechanical' protection [1]. Tubes were laid using mandrel plumbers' joints, which reduced the chance of leakage occurring. Many tubes lasted more than fifty years, the main requirement for replacement being as a result of damage inflicted by repairs to the road above. Most tubes were 2 1/2 inches in diameter, with 3 inch diameter tubes being used where traffic was particularly heavy. Messages were placed in carriers, each carrier holding between 20 an 30 messages. Carriers consisted of a cylinder of gutta percha, covered with felt sleeves which acted as skirts within the tube. A thick felt pad was fitted to front of the carrier to act as a buffer on arrival. The carrier was then placed into the tube either by inserting it through a funnel shaped end piece or by lifting a small flap door. Carriers could be dispatched every few seconds (in long tubes multiple carriers were allowed, alt
hough they had to be dispatched at regular intervals. The carrier was then drawn along the tube at an average speed of 30 feet per second, or 20 mph . Street tubes, which varied in length from a few hundred feet to over three miles, required pressures of up to 12 lb per sq inch above atmosphere in order to achieve these speeds. This pressure is created by supplying compressed air at one end of the tube, and leaving the other open to the atmosphere; or alternatively leaving one end open to the atmosphere and exhausting the air at the other by means of a suction pump which maintains partial vacuum in the tube. Initially air differentials were created by steam driven beam engines. By the 1930s, two electrically driven compressors operate all street tubes in London.

On arrival at their destination, carriers were transferred to a house tube system. Carriers were stopped by a grid in front of a 'rotary switch', which acted as the link between the street and house systems. Once stopped the carrier formed an obstruction to the free flow of air, causing a difference in pressure across the switch. This activated a differential indicator, which allowed the carrier to transfer between systems. Such a device was required, since house tube systems generally operated at lower pressures (not exceeding 1/2 lb per sq inch). House tubes ran in loops, to allow messages to both be sent and received. As well as acting as the final part of the system for moving telegram messages, house tubes also provided a system for passing 'tickets' (details of long distance calls) between telephone operators. The London system remained in use till 1962, when it was superseded by technological development such as telephone networks, telexes and teleprinters.

http://www.capsu.org/history/telegram_conveyors.html