Pneumatic Dispatch

PNEUMATIC DISPATCH, a system of transporting written dispatches through long tubes of small diameter by means of compressed or rarefied air. It was introduced in 1853 by J. Latimer Clark between the Central and Stock Exchange stations of the Electric and International Telegraph company in London, the stations being connected by a tube I lin. in diameter and 22oyd. long, the messages, enclosed in a tight fitting carrier, being drawn through it by the production of a partial vacuum at one end. The system was improved in 1858 by C. F. Varley, who used compressed air to return the carriers in the other direction. By this means it was possible to develop two-way working on single tubes between a central station, equipped with air-com pressing plant, and outlying offices.

Pneumatic dispatch tubes are in extensive use in many coun tries for both telegraphic and postal matter.

Radial System:

This system of pneumatic dispatch was de veloped by R. S. Culley and R. Sabine in connection with the British post office for the transmission of telegrams between local collection and delivery offices and the central telegraph office; it became generally more economical, under normal conditions of traffic and distance, to transmit these messages by tube rather than wire so dispensing with the employment of skilled telegraphists.

Since that time the system has been greatly extended both in London and the large provincial towns until in London alone the street tubes laid measure over 57m., varying in length from iooyd. to nearly 4,000yd. These tubes are either single tubes working in both directions or, where the traffic warrants it, sep arate "up" and "down" tubes are installed. In a few cases inter mediate offices are connected to the tubes, but this practice is not desirable as delay in transmission is caused and direct tubes are installed wherever possible.

A further development is the laying of pressure worked trunk tubes from the central telegraph office to an outlying centre, mes sages being transmitted between this centre, where a pumping plant is also installed, by radial tubes to offices in the vicinity.

House Tubes and Street Tubes.

Short tubes known as house tubes are in use in a large number of offices and telephone exchanges for carrying messages from the public counter or one room to another. These tubes, which are generally r tin. in diame ter, are made of brass and are operated by hand-worked pumps where the distance is short and the traffic inconsiderable, or by means of small electrically driven centrifugal fans or other form of blower. The pressure or vacuum required is only a few inches

as shown by water gauge and the blowers are either run continu ously or switched on as required.

Street tubes used by the post office are generally 24in. in diameter, but Sin. and z 2in. tubes are also used. These tubes when laid in the street are in all cases made of lead and are pro tected by cast-iron pipes. Where they are run in buildings or subways brass tubing is used. They are operated by electrically driven compressors.

Carriers.

The carriers, in which the messages are inserted for dispatch, are made of gutta-percha covered with felt, the front of the carrier being provided with a buffer or head formed by several layers of felt fitting the tube closely, the messages being held in place by means of an elastic band. The Sin. carriers hold 5o, the 2 4in. carriers 20 and the r tin. carriers five ordinary forms. The carrier used on house tube systems are generally made of fibre the messages being retained by a clip.

Working.

The air for working the street tubes is supplied by electrically driven compressors, the standard pressure and vacuum used being i olb. and 621b. per sq.in. respectively, which values give approximately the same speed.

The time of transit of a carrier through a tube at the ordinary pressures in use is given approximately by the empirical formula:— For a given transit time the horse-power required is less in the case of vacuum than in the case of pressure working, owing to the lesser density of the air column moved : thus, for example, the transit time under i olb. pressure is the same as with a vacuum of 611b., but the horse-power required is as 1-83 to i. A 24in. tube r m. long worked at iolb. per sq.in. pressure will have a transit time of 21min. and will theoretically require 3.35 h.p. to work it. Actually owing to various losses 25% more power must be allowed for the compressor. When working at the same pressure the transit time for a 24in. tube is 16% more than that for a corresponding length of Sin. tube, but the power required is 50% less and it is therefore advisable to use the smallest tube compatible with the traffic.