The Berlin Pneumatic Telegraph.
—In 1863 the Prussian government applied to the firm of Siemens and Halske, of Berlin, for a proposition to establish a system of pneumatic tubes in that city for the transmission of telegraph messages. A proposition was accordingly submitted, and the work was completed in 1865. This first line consisted of two parallel wrought-iron tubes, two and one-half inches in diameter, one tube being used exclusively to send in one direction, and the other in the opposite direction. They extended from the telegraph station to the Exchange, requiring a total length of five thousand six hundred and seventy feet of tube. The two tubes were looped together at the Exchange, and a continuous current of air was made to circulate in them by a double-acting steam air-pump, located at the telegraph station. Air was compressed into one end of the tube and exhausted from the other. With nine inches of mercury pressure and vacuum the passage was made in ninety-five seconds to the Exchange, and seventy-five seconds from the Exchange. It was similar to the line established in London by the same firm some years later, which we have already described, except that there was no intermediate station. After the line had been in use for a year and a half, the Prussian government had it extended, first, from the telegraph station to the Potsdam gate, with an intermediate station at the Brandenburg gate. After these preliminary experiments, further extensions were made until a net-work of tubes extended over the city of Berlin, including no less than thirty-eight stations and over twenty-eight miles of tubes; but in laying down this net-work a departure was made from the Siemens system. Air was no longer kept constantly circulating, but power was stored up in large tanks, some being exhausted and others filled with compressed air, which was used when required to send messages, usually at intervals of five or fifteen minutes. The exhausted tanks were permanently connected with the closed tubes, which were opened when required for use. The tanks containing compressed air were connected to the tubes when messages were sent. The internal diameter of the tubes was 2.559 inches. They were laid in circuits, including several stations in a circuit, and the carriers travelled only in one direction around the circuit. Some outlying stations were connected by a single tube with central pumping-stations, these single tubes being worked in both directions. Years of experience have shown the disadvantages of this circuit-system, and it has gradually been changed to the radial system, such as is used in London, until now nearly all the stations are grouped around the central pumping-stations, to which they are connected directly by radiating tubes. The Siemens apparatus has been replaced by simpler and less expensive valves and receiving-boxes, the latest form of which was designed and patented by Mr. Josef Wildemann.
Fig. 3.
DIAGRAM OF PART OF PARIS PNEUMATIC TUBE SYSTEM.
PARIS CIRCUIT SYSTEM.
Larger image (202 kB)
The Paris Pneumatic Telegraph.
—We will now glance at the system used in Paris, which has some novel features. In 1865 it was decided to establish a system, and the first experimental line, from Place de la Bourse to the Grand Hôtel, on the Boulevard des Capuciens, was laid in 1866. Instead of using a steam-engine to drive an air-compressor or exhaust-pump, air was compressed in tanks by displacement with water from the city mains. In 1867 this line was extended to Rue de Grennelle St. Germain, with an intermediate station at the Rue Boissy d’Anglais, and another line with stations at Rue Jean Jacques Rousseau, Hôtel du Louvre in the Rue de Rivoli, the Rue des Saints Pères, and terminating in the central station. In 1868 the system was changed to a polygonal or circuit system by removing the station in the Rue de Rivoli to the Place du Théâtre Français and connecting the latter directly with the Bourse. Other changes and extensions were made in 1870 and 1871, until three polygons or circuits were formed, with five or six stations in each circuit, and several outlying stations were connected by independent tubes. In the middle of the year 1875 seventeen stations had been connected and plans were made for twenty-one more. Instead of maintaining an air-current around each circuit by machinery located at one of the stations on the circuit, at least three of the five or six stations comprised in the circuit have means of supplying compressed air or of exhausting it, and each side of the polygon, or section of the circuit between two stations, is operated independently of the rest of the circuit (see Fig. 3). Carriers are sent on from station to station around the circuit, either by compressed air from the last station from which they were sent or by means of exhaustion at the next station towards which they are moving. The carriers are made up in trains of from six to ten, with a piston behind them that fits the tube closely and forces them ahead. Each carrier is addressed by means of a label for its destined station. Trains are despatched around the circuits at stated times, usually at fifteen-minute intervals. As they arrive at the various stations, carriers are taken out and others put in, and the trains sent on their way. The carriers consist of iron cylinders, closed at one end, with a leather case that slides over them and closes the open end. They weigh, when filled with thirty-five messages, twelve and one-half ounces, and they will travel about twelve hundred miles before the leather cover is so worn that it must be thrown away. The pistons are made of a wooden cone, covered with iron, and having a “cup-leather” upon the rear end that fits the tube closely. The sending and receiving apparatus consists of sections of tube closed at one end, having a door on the side, through which carriers are inserted or despatched. A peculiar form of fork is used for picking them out. The air is controlled by valves opened and closed by hand.
Several methods are used to compress and exhaust the air. The most novel method consists in having tanks in which a partial vacuum is produced by allowing water to flow out of them into the sewer, or in having the air compressed by allowing water from the city mains to flow into the tanks and displace the air. Water jets have also been used, operating similar to a steam-injector. At some of the stations water turbines drive the air-pumps, and at others steam-engines are used.
The tubes of the Paris system are of wrought iron, in lengths of from fifteen to twenty feet, the joints being made with flanges and bolts. The interior diameter is 2.559 inches with a maximum variation to 2.519 inches. The bends are made with a radius of from six to one hundred and fifty feet. Water frequently gives trouble by accumulating in the tubes. Traps are placed at low points to drain it off.