A Story of the Telegraph

by John Murray

Cable Instruments.

The apparatus in use on land lines are not adapted for cables except for comparatively short distances not exceeding four or five hundred miles.

When the Atlantic cable was laid a special instrument had to be devised to transmit signals to the distant end. The man to accomplish this was Professor Thomson (now Lord Kelvin), who invented the mirror system. A beam of light was thrown on a minute mirror an eighth of an inch in diameter and the light reflected on to a scale by means of which the signal was interpreted into letters. This necessitated one person constantly scanning the spot of light as it moved to the right and to the left of the scale and calling out the individual letters, which were taken down by another person. This tedious and trying method of receiving signals was superceded by another device of Lord Kelvin, the siphon recorder.

The siphon, by which the cable signals are automatically recorded, is a thin glass tube, about the thickness of a strong linen thread, and quite flexible. It is suspended in a frame and attached by a single silk fibre to one side of a rectangular coil of fine insulated wire, moving about a soft iron bar fixed in the magnetic field of two large permanent magnets. The coil is held down at the lower end by a silk thread, fastened to an adjustable spring, to regulate or confine the lateral motion of the siphon, the magnets are placed vertically and are two inches apart, one end of the siphon is twice bent at right angles, and dips into an ink well filled with filtered aniline ink. The other end has a minute thread or short piece of soft iron cemented longitudinally to it, and sways in close proximity to a narrow fillet of paper five-eighths of an inch wide, which is drawn along by a small motor. The small motors by which the paper is drawn along receive their current generally from lead-lined trays, 18 by 20 inches, at the bottom of which is placed a copper sheet, the zinc is wrapped in stout manilla paper which serves the purpose of a porous cup for the sulphate of copper. The cable current passes through the small rectangular coil, which is about two inches long, as both positive and negative currents are sent into the condensers, and thereby disturb the static electricity of the cable. The coil is deflected to the right and left respectively, tending to place itself at right angles to the lines of magnetic force between the fixed bar magnets and which lines of force are concentrated by the small bar above mentioned of the best soft iron within the coil. The siphon has, therefore, a corresponding motion to the coil. As the mechanical force of the suspended coil is very small in deflecting, it is necessary that the siphon be not in continuous contact with the fillet of paper otherwise its motion would cease. The difficulty of obtaining a record is overcome in an ingenious manner. The siphon is made to vibrate by means of a local battery on the principle of the push button electric bell by the breaking of the circuit—the vibration is communicated to the siphon by the interposition of another electro-magnet in the local circuit and placed underneath the fillet of paper, the small thread of iron on the tip of the siphon acts as the armature to the latter electro-magnet. The number of vibrations made in a second depends on the siphon, different siphons having different periods or inherent notes, but 55 is about the number of vibrations a second, every pulsation of the siphon deposits a drop of ink on the paper, and, as the paper is moving at the rate of over half an inch a second, an apparently continuous line is drawn.

From the description of the working of the siphon—of its lateral movements—it will be evident that the cablegram, as shown on the fillet of paper, will look like the contour line across the Rocky Mountains. The undulations made by the siphon correspond to the clicking we hear in the ordinary telegraph instruments. A cable office is very quiet compared to the bewildering clatter in a large telegraph office.

It was found that on the Atlantic (and shorter cables) a greater speed of signals was possible than could be sent through by hand with the double key. This called forth the invention of the so-called automatic transmitter.

For this purpose the messages are in the first place punched into a strip of oiled and prepared paper, the characters on the strip are represented by holes at varying distances on each side of a central line. This strip of continuous paper is then fed into the transmitter, in which metallic points slide along the under side of the strip. Wherever a hole is encountered electric contact is made and a signal sent. The speed of running the strip through the transmitter can be regulated as desired.

The “auto” can easily keep two men busy punching.

Within recent years an improvement has been effected for transmitting signals or messages automatically from one cable to another. Formerly it was necessary after receiving the signals from one cable to transmit them by hand to the connecting cable at the station. Now, however, this can be done automatically by means of Taylor, Brown and Dearlove’s Translator. The siphon in it instead of carrying ink contains a metallic thread which rests, instead of on the fillet of paper, on a rapidly, revolving, perfectly, smooth, small wheel, in which the surface of the circumference is divided into three parts, the central one known as “no man’s land” being a non-conductor such as glass, while the outer ones are of silver. As the siphon sways to one side or the other it makes metallic contact, which is communicated by means of “brushes” which press against each side of the wheel to the outgoing cable.

This translator simplifies the work and reduces the office staff which would be otherwise necessary.