Pleasant Ways in Science

by Richard A. Proctor

Fig. 6.

If the insulated wire is twisted in the form of a helix or coil round a bar of soft iron, the bar becomes magnetized while the current is passing. If the bar be bent into the horse-shoe form, as in [Fig. 6], where A C B represents the bar, a b c d e f the coil of insulated wire, the bar acts as a magnet while the current is passing along the coil, but ceases to do so as soon as the current is interrupted.[30] If, then, we have a telegraphic wire from a distant station in electric connection with the wire a b c, the part e f descending to an earth-plate, then, according as the operator at that distant station transmits or stops the current, the iron A C B is magnetized or demagnetized. The part C is commonly replaced by a flat piece of iron, as is supposed to be the case with the temporary magnets shown in [Fig. 7], where this flat piece is below the coils.

So far back as 1838 this property was applied by Morse in America in the recording instrument which bears his name, and is now (with slight modifications) in general use not only in America but on the Continent. The principle of this instrument is exceedingly simple. Its essential parts are shown in [Fig. 7]; H is the handle, H b the lever of the manipulator at the station A. The manipulator is shown in the position for receiving a message from the station B along the wire W. The handle H´ of the manipulator at the station B is shown depressed, making connection at a´ with the wire from the battery N´ P´. Thus a current passes through the handle to c´, along the wire to c and through b to the coil of the temporary magnet M, after circling which it passes to the earth at e and so by E´ to the negative pole N´. The passage of this current magnetizes M, which draws down the armature m. Thus the lever l, pulled down on this side, presses upwards the pointed style s against a strip of paper p which is steadily rolled off from the wheel W so long as a message is being received. (The mechanism for this purpose is not indicated in [Fig. 7].) Thus, so long as the operator at B holds down the handle H´, the style s marks the moving strip of paper, the spring r, under the lever s l, drawing the style away so soon as the current ceases to flow and the magnet to act. If he simply depresses the handle for an instant, a dot is marked; if longer, a dash; and by various combinations of dots and dashes all the letters, numerals, etc., are indicated. When the operator at B has completed his message, the handle H´ being raised by the spring under it (to the position in which H is shown), a message can be received at B.

Fig. 7.

I have in the figure and description assumed that the current from either station acts directly on the magnet which works the recording style. Usually, in long-distance telegraphy, the current is too weak for this, and the magnet on which it acts is used only to complete the circuit of a local battery, the current from which does the real work of magnetizing M at A or M´ at B, as the case may be. A local battery thus employed is called a relay.

The Morse instrument will serve to illustrate the principle of the methods by which facsimiles are obtained. The details of construction are altogether different from those of the Morse instrument; they also vary greatly in different instruments, and are too complex to be conveniently described here. But the principle, which is the essential point, can be readily understood.

In working the Morse instrument, the operator at B depresses the handle H´. Suppose that this handle is kept depressed by a spring, and that a long strip of paper passing uniformly between the two points at a prevents contact. Then no current can pass. But if there is a hole in this paper, then when the hole reaches a the two metal points at a meet and the current passes. We have here the principle of the Bain telegraph. A long strip of paper is punched with round and long holes, corresponding to the dots and marks of a message by the Morse alphabet. As it passes between a metal wheel and a spring, both forming part of the circuit, it breaks the circuit until a hole allows the spring to touch the wheel, either for a short or longer time-interval, during which the current passes to the other station, where it sets a relay at work. In Bain’s system the message is received on a chemically prepared strip of paper, moving uniformly at the receiving station, and connected with the negative pole of the relay battery. When contact is made, the face of the paper is touched by a steel pointer connected with the positive pole, and the current which passes from the end of the pointer through the paper to the negative pole produces a blue mark on the chemically prepared paper.[31]

We see that by Bain’s arrangement a paper is marked with dots and lines, corresponding to round and elongated holes, in a ribbon of paper. It is only a step from this to the production of facsimiles of writings or drawings.