In this the generator G and its commutator disk 3, with the various brushes, 1, 2, 4, and 5, are arranged in the same manner as is shown in Fig. 171.  It is evident from what has been said that wire 6 leading from generator brush 2 and commutator disk 3 will carry alternating potential; that wire 7 will carry positive pulsations of potential; and that wire 8 will carry negative pulsations of potential.  There are five keys in the set illustrated in Fig. 174, of which four, viz, K^{1}, K^{2}, K^{3}, and K^{4}, are connected in the same manner as diagrammatically indicated in Figs. 172 and 173, and will, obviously, serve to send the proper current over the proper limb of the line to ring one of the bells.  Key K^{5}, the fifth one in the set, is added so as to enable the operator to ring an ordinary unbiased bell on a single party line when connection is made with such line.  As the two outside contacts of this key are connected respectively to the two brushes of the alternating-current dynamo G, it is clear that it will impress an alternating current on the line when its contacts are closed.

Circuits of Two-Party Line Telephones. In Fig. 175 is shown in detail the wiring of the telephone set usually employed in connection with the party-line selective-ringing system illustrated in Fig. 170.  In the wiring of this set and the two following, it must be borne in mind that the portion of the circuit used during conversation might be wired in a number of ways without affecting the principle of selective ringing employed; however, the circuits shown are those most commonly employed with the respective selective ringing systems which they are intended to illustrate.  In connecting the circuits of this telephone instrument to the line, the two line conductors are connected to binding posts 1 and 2 and a ground connection is made to binding post 3.  In practice, in order to avoid the necessity of changing the permanent wiring of the telephone set in connecting it as an A or B Station (Fig. 170), the line conductors are connected to the binding posts in reverse order at the two stations; that is, for Station A the upper conductor, Fig. 170, is connected to binding post 1 and the lower conductor to binding post 2, while at Station B the upper conductor is connected to binding post 2 and the lower conductor to binding post 1.  The permanent wiring of this telephone set is the same as that frequently used for a set connected to a line having only one station, the proper ringing circuit being made by the method of connecting up the binding posts.  For example, if this telephone set were to be used on a single station line, the binding posts 1 and 2 would be connected to the two conductors of the line as before, while binding post 3 would be connected to post 1 instead of being grounded.