In the operation of the machine, the switch lever is not moved its full throw at first, but must lie held for a moment in an intermediate position. This is necessary in order that no mistake shall be made in the clearing of the signals. The first movement of the switch lever operates the valve which moves the switch. Durin the movement of the switch the indication circuit is temporarily closed, thereby releasing one portion of the lock on the back of the switch roller. Leon the com pletion of the movement of the switch the indication circuit is again broken, and permits the operator to com plete the throw of the switch lever. The only communi cation between the tower and the different switches and signals is by insulated copper wire. In the smaller machines a gravity battery is used to furnish the current, but in the largest recent machines the current is taken directly from a storage battery. The signal movements used in the pneu matic interlocking are the same as those used in the pneumatie block signaling. which have already been described. The Pneumatic Switch and Cylinder is illustrated in horizon ttkl section in Fig. 14, and in external appearance, together with the switch and lock move ment, in Fig. 15. The outside magnets, A and 0, control alternately, depending on the position of the lever in the tower, the admission of air into the valve cylinder. The central magnet,
B. controls the valve lock, By moving a switch lever in the tower, the following operation takes place : The magnet, B, is first charged (it is so shown in the drawing), which admits air into the lock cylinder and releases the sliD valve, leaving it free to move as soon as the pressure shall be applied to it from cylinder I. Magnet C is then charged, and magnet A is discharged, permitting the entrance of air into cylinder 1 and opening the ex haust port of cylinder 2. This forces over the slide valve to its other position, allowing the entrance of pressure to the right-hand side of the main cylinder, and connecting the left-hand side of the main eyl inder with the atmosphere. The last movement of the lever in the tower cuts the current out of the magnet B, thereby locking up the slide valve in its new position.
The switch movement shown in Fig. 15 is the same as that described under the head of mechanieal interlocking. A pin in the slide bar transmits the power to the wide jaw to which the switch is con nected The detector bar and lock, however, are connected directly to the slide bar, and move during its whole stroke, while the switch Moves only the middle part of the stroke.