Pleasant Ways in Science

by Richard A. Proctor

Now suppose that P and N, [Fig. 2], are the positive and negative poles of a galvanic battery at A, and that a wire passes from P to the station B, where it is coiled round a needle suspended vertically at n, and thence passes to the negative pole N. Let the wire be interrupted at a b and also at c d. Then no current passes along the wire, and the needle n remains at rest in a vertical position. Now suppose the points a b connected by the wire a b, and at the same moment the points c d connected by the wire c d, then a current flows along P a b to B, as shown in [Fig. 2], circuiting the coil round the needle n and returning by d c to N. The upper end of the needle is deflected to the right while this current continues to flow; returning to rest when the connection is broken at a b and c d. Next, let c b and a d be simultaneously connected as shown by the cross-lines in [Fig. 3]. (It will be understood that a d and b c do not touch each other where they cross.) The current will now flow from P along a d to B, circuiting round the needle n in a contrary direction to that in which it flowed in the former case, returning by b c to N. The upper end of the needle is deflected then to the left while the current continues to flow along this course.

I need not here describe the mechanical devices by which the connection at a b and c d can be instantly changed so that the current may flow either along a b and d c, as in [Fig. 2], circuiting the needle in one direction, or along a d and b c, as in [Fig. 3], circuiting the needle in the other direction. As I said at the outset, this paper is not intended to deal with details of construction, only to describe the general principles of telegraphic communication, and especially those points which have to be explained in order that recent inventions may be understood. The reader will see that nothing can be easier than so to arrange matters that, by turning a handle, either (1), a b and c d may be connected, or, (2), a d and c b, or, (3), both lines of communication interrupted. The mechanism for effecting this is called a commutator.

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].

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 handle of the commutator at A is turned in the other direction, b and c are connected, as also a and d; the current from P passes along a d to the ground plate G, thence to G´, along d´ b´, round the needle n´, back by the wire to the station A, where, after circuiting the needle n in the same direction as the needle n´, it travels by b and c to the negative pole N. The upper end of the needle, at both stations, is deflected to the left by the passage of the current in this direction.

It is easily seen that, with two wires and one battery, two needles can be worked at both stations, either one moving alone, or the other alone, or both together; but for the two to move differently, two batteries must be used. The systems by which either the movements of a single needle, or of a pair of needles, may be made to indicate the various letters of the alphabet, numerals, and so on, need not here be described. They are of course altogether arbitrary, except only that the more frequent occurrence of certain letters, as e, t, a, renders it desirable that these should be represented by the simplest symbols (as by a single deflection to right or left), while letters which occur seldom may require several deflections.

One of the inventions to which the title of this paper relates can now be understood.