40 m.), whilst in all large and also in very many smaller
provincial towns there are installations; these are constantly
being added to, as it is found more economical to transmit local
message-work by tube rather than by wire, as skilled telegraphists
are not required, but only tube attendants. In some cases
only a single tube is necessary, but three or four, or even more,
are in use in some towns, according to local circumstances.
Short tubes, known as “house tubes” are in use in a great
number of offices; such tubes, which are worked either by hand pumps
(when the tubes are very short and the traffic inconsiderable)
or by power, are usually 112 in. in diameter, and
are used for the purpose of conveying messages from one
part of a telegraph instrument-room to another, or from the
instrument-room to the public counter. The underground,
or “street” tubes are chiefly 214 in. in diameter, but there are
also a number of 3-in. tubes in use; those in the large provincial
towns (Birmingham, Bradford, Cardiff, Edinburgh, Glasgow,
Grimsby, Liverpool, Manchester, Newport, Leeds, Newcastle,
Southampton and Swansea) are 214 in. in diameter; but in
Dublin, Gloucester, Lowestoft and Milford 112-in. tubes are
employed. There are fifty street tubes in London, varying in
length from 100 to 2000 yds (central office to the Houses of
Parliament), and also seventy-five house tubes; the pumps
for the whole system are worked by four 100 horse-power
steam-engines. At Cardiff, Edinburgh, Gloucester, Leeds,
Lowestoft, Newport, Southampton and Swansea the pumps
are driven by electric motors; at Bradford and Grimsby gas-engines
are used, and at Milford an oil-engine.
The tubes are in all cases of lead, the 214-in. tubes weighing 8 ℔ per foot run and being made in lengths of 28 ft.; they are enclosed in 3-in. cast-iron pipes made in lengths of 9 ft.
Great care is exercised in making the joints in the lead pipes. Before the tube is placed in its trench a strong chain is passed through it, and a polished steel mandrel, 6 in. long and slightly less in diameter than the diameter of the tube, is heated and attached to the chain, and pushed half its length into the end of the tube already laid; the new length of tube is then forced over the projecting end of the mandrel until the tube ends (which have been previously cut flat) butt perfectly together; an ordinary plumber’s joint is then made. By this means the tube is made perfectly air-tight, and the mandrel keeps the surface of the tube under the joint as smooth as at any other part of its length. After the joint is completed the mandrel is drawn out by the chain attached to it, the next len th is drawn on, and the above process repeated. The tubes are gaid about 2 ft. below the surface of the ground.
The tubes radiate from the central to the branch offices, the principal offices having two tubes, one for “inward” and the other for “outward” traffic. At the smaller offices both the inward and the outward traffic is carried on through one tube. The “carriers” are made with guttapercha bodies, covered with felt, the front of the carrier beingWorking. provided with a buffer or piston formed of several disks of felt which closely fit the tube; the messages are prevented from getting out of the carrier by the end being closed by an elastic band, which can be stretched sufficiently to allow the message forms to be inserted. The 3-in. carriers will hold 75 ordinary message forms, the 214-in. carriers 25 forms, and the 112-in. carriers 20 forms. The carriers are propelled in one direction (from the central office) by “pressure,” and drawn in the opposite direction by “vacuum,” the standard pressure and vacuum being io Tb and 612 ℔ per sq. in. respectively, which values give approximately the same speed.
The time of transit of a carrier through a tube when the air pressure does not exceed 20 ℔ per square inch is given very approximately by the empirical formula:—
;
where l=length of tube in yards, d=diameter of tube in inches, P=effective air-pressure in pounds per square inch, t=transit time in seconds. For vacuum the formula is:—
where P1＝effective vacuum in pounds per square inch.
The horse-power required to propel a carrier is approximately, for pressure:—
H.P.＝(·574+·0011P),
for vacuum:—
H.P.＝(5·187 − 1·214
For a given transit time the actual horse-power required is much less in the case of vacuum than in the case of pressure working, owing to the density of the air column moved being much less: thus, for example, the transit time for 10 ℔ pressure is the same as for 612 ℔ vacuum, but the horse-power required in the two cases is as 1·83 to 1. A tube 1 m. long, 214 in. in diameter, and worked at 10 ℔ per square inch pressure, will have a transit time of 214 minutes, and will theoretically require 3·35 horse-power to be expended in working it, although actually 25% more horse-power than this must be allowed for, owing to losses through various causes. The transit time for a 214-in. tube is 16% more than for a 3-in. tube of the same length, when both are worked at the same pressure, but the horse-power required is 50% less; it is not advisable, therefore, to use a tube larger than is absolutely necessary to carry the volume of traffic required.
The somewhat complicated pattern of “double sluice valve” originally used at the central stations has been superseded by a simpler form, known as the “D” box-so named from the shape of its cross section. This box is of cast iron, and is provided with a close-fitting, brass-franied, sliding iid with a glass panel. ThisDespatching and Receiving Apparatus. lid fits air-tight, and closes the box after a carrier has been inserted into the mouth of the tube; the latter enters at one end of the box and is there bell-mouthed. A supply pipe, to which is connected a “3-way” cock, is joined on to the box and allows communication at will with either the “pressure” or “vacuum” mains, so that the apparatus becomes available for either sending (by pressure) or receiving (by vacuum) a carrier. Automatic working, by which the air supply is automatically turned on on the introduction of the carrier into a tube and on closing of the D box, and is cut off when the carrier arrives, was introduced in 1909.
On the long tubes (over about 1000 yds.) a modification of the “D” box in its simplest form is necessary; this modification consists in the addition of a “sluice” valve placed at a distance of about 9 in. (i.e. rather more than the length of a carrier) from the mouth of the tube. The sluice valve, by means of an interlocking arrangement, is so connected with the sliding lid of the box that the lid cannot be moved to the open position unless the sluice valve has closed the tube, nor can the sluice valve be opened unless the sliding lid is closed. The object of this sluice valve is to prevent the back rush of air which would take place into the tube when the sliding lid is opened to take out a carrier immediately on the arrival of the latter; for although the vacuum may be turned off by the 3-way cock, yet, owing to the great length of the tube, equilibrium does not immediately take place in the latter, and the back rush of air into the vacuum when the lid is opened to extract the carrier will cause the latter to be driven back into the tube. The sluice also prevents a similar, but reverse, action from taking place when pressure working is being carried on.
As a rule, only one carrier is dispatched at a time, and no second carrier is inserted in the tube until the arrival of the first one at the farther end is automatically signalled (by an electric apparatus) to the dispatching office. On some of the long tubes a carrier, when it passes the midway point in the tube, strikes a trigger and sends back an electrical signal indicating its passage; on the receipt of this signal a second carrier may be despatched. This arrangement has been almost entirely superseded by a signalling apparatus which by a clock movement actuates an indicating hand and moves the latter to “tube clear” a certain definite time (30 to 40 seconds) after a carrier has been inserted in the tube. By this arrangement carriers can be dispatched one after the other at comparatively short intervals of time, so that several carriers (separated by distinct intervals) may be travelling through the tube simultaneously. It is necessary that the carriers be separated by a definite interval, otherwise they tend to overtake one another and become jammed
