generator G^{2} to pass over the upper side of the line through the bell and condenser at Station B and return by the path through the ground.  The object of grounding the opposite sides of the keys at the central office is to prevent cross-ringing, that is, ringing the wrong bell.  Were the keys not grounded this might occur when a ringing current was being sent out while the receiver at one of the stations was off its hook; the ringing current from, say, generator G^{1} then passing not only through the bell at Station A as intended, but also through the bell at Station B by way of the bridge path through the receiver that happened to be connected across the line.  With the ringing keys grounded as shown, it is obvious that this will not occur, since the path for the ringing current through the wrong bell will always be shunted by a direct path to ground on the same side of the line.

In such a two-party-line selective system the two generators G^{1} and G^{2} may be the same generator and may be of the ordinary alternating-current type.  The bells likewise may be of the ordinary alternating-current type.

The two-party selective line just described virtually employs two separate circuits for ringing.  Now each of these circuits alone may be employed to accomplish selective ringing between two stations by using two biased bells oppositely polarized, and employing pulsating ringing currents of one direction or the other according to which bell it is desired to ring.  One side of a circuit so equipped is shown in Fig. 171.  In this the two biased bells are at Station A and Station B, these being bridged to ground in each case and adapted to respond only to positive and negative impulses respectively.  At the central office the two keys K^{1} and K^{2} are shown.  A single alternating-current generator G is shown, having its brush 1 grounded and brush 2 connected to a commutator disk 3 mounted on the generator shaft so as to revolve therewith.  One-half of the periphery of this disk is of insulating material so that the brushes 4 and 5, which bear against the disk, will be alternately connected with the disk and, therefore, with the brush 2 of the generator.  Now the brush 2, being one terminal of an alternating-current machine, is alternately positive and negative, and the arrangement of the commutator is such that the disk, which is always at the potential of the brush 2, will be connected to the brush 5 only while it is positively charged and with the brush 4 only while it is negatively charged.  As a result, brush 5 has a succession of positive impulses and brush 4 a succession of negative ones.  Obviously, therefore, when key K^{1} is depressed only the bell at Station A will be rung, and likewise the depression of key K^{2} will result only in the ringing of the bell at Station B.