open beyond.  Sending a single preliminary impulse will, however, operate a switching device so as to disconnect the bell at the first station and to connect the line through to the second station.  This may be carried out, by sending the proper number of preliminary impulses, so as to build up the line circuit to the desired station, after which the sending of the ringing current will cause the bell to ring at that station only.

Polarity Method.  The polarity method of selective signaling on party lines is probably the most extensively used.  The standard selective system of the American Telephone and Telegraph Company operates on this principle.

Two-Party Line. It is obvious that selection may be had between two parties on a single metallic-circuit line without the use of biased bells or current of different polarities.  Thus, one limb of a metallic circuit may be used as one grounded line to ring the bell at one of the stations, and the other limb of the metallic circuit may be used as another grounded line to ring the bell of the other station; and the two limbs may be used together as a metallic circuit for talking purposes as usual.

This is shown in Fig. 170, where the ringing keys at the central office are diagrammatically shown in the left-hand portion of the figure as K^{1} and K^{2}.  The operation of these keys will be more fully pointed out in a subsequent chapter, but a correct understanding will be had if it be remembered that the circuits are normally maintained by these keys in the position shown.  When, however, either one of the keys is operated, the two long springs may be considered as pressed apart so as to disengage the normal contacts between the springs and to engage the two outer contacts, with which they are shown in the cut to be disengaged.  The two outer contacts are connected respectively to an ordinary alternating-current ringing generator and to ground, but the connection is reversed on the two keys.

[Illustration:  Fig. 170.  Simple Two-Party Line Selection]

At Station A the ordinary talking set is shown in simplified form, consisting merely of a receiver, transmitter, and hook switch in a single bridge circuit across the line.  An ordinary polarized bell is shown connected in series with a condenser between the lower limb of the line and ground.  At Station B the same talking circuit is shown, but the polarized bell and condenser are bridged between the upper limb of the line and ground.

If the operator desires to call Station A, she will press key K^{1} which will ground the upper side of the line and connect the lower side of the line with the generator G^{1}, and this, obviously, will cause the bell at Station A to ring.  The bell at Station B will not ring because it is not in the circuit.  If, on the other hand, the operator desires to ring the bell at Station B, she will depress key K^{2}, which will allow the current from