Two points remain, however, to be explained: First, A must be a receiving station as well as a transmitting station; secondly, the wire connecting B with N, in Figs. 2 and 3, can be dispensed with, for it is found that if at B the wire is carried down to a large metal plate placed some depth underground, while the wire at _c_ is carried down to another plate similarly buried underground, the circuit is completed even better than along such a return wire as is shown in the figures. The earth either acts the part of a return wire, or else, by continually carrying off the electricity, allows the current to flow continuously along the single wire. We may compare the current carried along the complete wire circuit, to water circulating in a pipe extending continuously from a reservoir to a distance and back again to the reservoir. Water sucked up continuously at one end could be carried through the pipe so long as it was continuously returned to the reservoir at the other; but it could equally be carried through a pipe extending from that reservoir to some place where it could communicate with the open sea—the reservoir itself communicating with the open sea—an arrangement corresponding to that by which the return wire is dispensed with, and the current from the wire received into the earth. The discovery that the return wire may be dispensed with was made by Steinheil in 1837. The actual arrangement, then, is in essentials that represented in Fig. 4. [Illustration: FIG. 4.] A and B are the two stations; P N is the battery at A, P´ N´ the battery at B; P´ P´ are the positive poles, N´ N´, the negative poles. At _n_ is the needle of station A, at _n´_ the needle of station B. When the handle of the commutator is in its mean position—which is supposed to be the case at station B—the points _b´ d´_ are connected with each other, but neither with _a´_ nor _c´_; no current, then, passes from B to A, but station B is in a condition to receive messages. (If _b´_ and _d´_ were not connected, of course no messages could be received, since the current from A would be stopped at _b´_—which does not mean that it would pass round _n´_ to _b´_, but that, the passage being stopped at _b´_, the current would not flow at all.) When (the commutator at B being in its mean position, or _d´ b´_ connected, and communication with _c´_ and _a´_ interrupted) the handle of the commutator at A is turned from its mean position in _one_ direction, _a_ and _b_ are connected, as are _c_ and _d_—as shown in the figure—while the connection between _b_ and _d_ is broken. Thus the current passes from P by _a_ and _b_, round the needle _n_; thence to station B, round needle _n´_, and by _b´_ and _d´_, to the earth plate G´; and so along the earth to G, and by _d c_, to the negative pole N. The upper end of the needle of both stations is deflected to the right by the passage of the current in this direction. When the