Fig. 86.
It is also possible, as shown at [Fig. 87], to send signals from two stations, using but one battery (which, if the distance is great, should be of a proportionate number of cells), two bells, and two ordinary pushes. Three
wires, besides the earth-plate or return wire, are required in this case. The whole of the wires, except the return, must be carefully insulated. Suppose in this case we press the right-hand button. The current flows from the battery along the lower wire through this right-hand push and returns to the distant bell along the top wire, down the left-hand dotted wire back to the battery, since it cannot enter by the left-hand press, which, not being pushed, makes no contact. The left-hand bell therefore rings. If, on the other hand, the left-hand push be pressed, the current from the carbon of the battery passes through the left-hand push, traverses the central line wire, passes into the bell, rings it, and descends to the right-hand earth plate E, traverses the earth circuit till it reaches the left-hand earth plate E, whence it returns to the zinc pole of the battery by the lower dotted line.
Fig. 87.
[Fig. 88] shows how the same result (signalling in both directions) may be attained, using only two wires, with earth return, and two Morse keys. The direction of the current is shown by the arrows. Both wires must be insulated and
either carried overhead or underground, buried in tubes. [Fig. 89] shows the proper mode of connecting the entire system of bells, pushes, etc., running through a building. The dotted lines are the wires starting from the two poles of the battery (which should consist of more cells in proportion as there is more work to do), the plain lines being the wires between the pushes and the bell and signalling box. In this illustration a door-pull is shown to the extreme left. Pendulum indicators are usually connected up as shown in this figure, except that the bell is generally enclosed in the indicator case. The wire, therefore, has to be carried from the left-hand screw of the indicator case direct to the upper dotted line, which is the wire returning to the zinc pole of the battery. N.B.—When the wires from the press-buttons are connected with the binding-screw, of the top of or inside of the indicator case, the insulating material of the wires, at the point where connection is to be made, must be removed, and the wires carefully cleaned and tightly clamped down.