Fig. 32.
ELECTRO-PNEUMATIC CIRCUIT-CLOSER.
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The Electro-Pneumatic Circuit-Closer.
—There is one piece of mechanism used in connection with the sending apparatus that we have yet to describe, and that is the circuit-closing device located in the manholes in the street. Since the carriers travel at a high rate of speed, they should not be made to operate any mechanism by impact with fingers or levers protruding into the tube when it can be avoided, even though the work to be done is so slight as the closing of an electric circuit, for the repeated impacts cannot fail to work injury to the carriers and the mechanism to be operated, no matter how carefully they are designed. To avoid such impacts, we have designed the electro-pneumatic circuit-closer, shown by the drawing in Fig. 32. It is operated by a passing carrier, but pneumatically rather than mechanically. In the figure we have a pneumatic tube, A, A, in which a carrier, B, is moving in the direction indicated by the arrow. At two points, about twenty or thirty feet apart, two small holes are tapped into the tube and pipes, C and D, are screwed in. These pipes lead to two chambers in a cast-iron box, F, separated by a diaphragm, E. This diaphragm is insulated electrically from the box supporting it, and is connected with the wire G. Just out of contact with the diaphragm is an insulated screw, H, connected with the wire I. These wires lead to the time-lock, already described, on the sending apparatus at the station. When no carrier is passing, the air-pressure is the same on both sides of the diaphragm, but when a carrier enters that part of the tube between the two points where the pipes C and D are connected, the equality of pressure on opposite sides of the diaphragm is destroyed. There is always a slightly greater pressure in rear of the carrier than in front of it, equal to the frictional resistance of the carrier in the tube. It is this difference of pressure in front and in rear of the carrier that moves it through the tube. When the carrier is in the position shown in the figure, the same difference of pressure will exist on opposite sides of the diaphragm, and it will be deflected into contact with the screw H, thereby closing the electric circuit. When the carrier has passed, equality of pressure on opposite sides of the diaphragm is established and the diaphragm takes its normal position, out of contact with the screw H. This apparatus is easily attached to the tube, and it contains no mechanism to get out of order.
Fig. 33.
OPEN RECEIVER.
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The Open Receiver.
—Wherever the pressure in the tube is down nearly to atmospheric, we can use an open receiver to discharge the carriers from the tube. This is a receiver that opens the tube to the atmosphere and allows the carrier to come out. Such a receiver is used at the main post-office in the Philadelphia postal-line, and was described in the last chapter. The present receiver is similar in operation, but contains some improvements in details. Fig. 33 is a side elevation of the apparatus, Fig. 34 is a longitudinal section, and Fig. 35 is a cross-section through the cylinder and valve, showing the sluice-gate.