In order to obviate the necessity for poling and also to bring about other desirable features, it has been, until recently, almost universal practice to so arrange the receiver that it would be in the circuit of the voice currents passing over the line, but would not be traversed by direct currents, this condition being brought about by various arrangements of condensers, impedance coils, or induction coils, as will be shown later.  During the year 1909, however, the adoption by several concerns of the so-called “direct-current” receiver has made it necessary for the direct current to flow through the receiver coils in order to give the proper magnetization to the receiver cores, and this has brought about a return to the very simple form of substation circuit, which includes the receiver and the transmitter directly in the circuit of the line.  This illustrates well an occurrence that is frequently observed by those who have opportunity to watch closely the development of an art.  At one time the conditions will be such as to call for complicated arrangements, and for years the aim of inventors will be to perfect these arrangements; then, after they are perfected, adopted, and standardized, a new idea, or a slight alteration in the practice in some other respect, will demand a return to the first principles and wipe out the necessity for the things that have been so arduously striven for.

[Illustration:  Fig. 130.  Bridging Battery with Repeating Coil]

Bridging Battery with Repeating Coil. As pointed out, the placing of the battery in series in the line circuit in the central office is not desirable, and, so far as we are aware, has never been extensively used.  The universal practice, therefore, is to place it in a bridge path across the line circuit, and a number of arrangements employing this basic idea are in wide use.  In Fig. 130 is shown the standard arrangement of the Western Electric Company, employed by practically all the Bell operating companies.  In this the battery at the central office is connected in the middle of the two sides of a repeating coil so that the current from the battery is fed out to the two connected lines in multiple.

Referring to the middle portion of this figure showing the central-office apparatus, 1 and 2 may be considered as the two halves of one side of a repeating coil divided so that the battery may be cut into their circuit.  Likewise, 3 and 4 may be considered as the two halves of the other side of the repeating coil similarly divided for the same purpose.  The windings of this repeating coil are ordinarily alike; that is, 1 and 2 combined have the same resistance, number of turns, and impedance as 3 and 4 combined.  The two sides of this coil are alternately used as primary and secondary, 1 and 2 forming the primary when Station A is talking, and 3 and 4, the secondary; and vice versa when Station B is talking.