Page:Popular Science Monthly Volume 9.djvu/92

74 In every form of electric telegraph the signals are given by an intermittent now of electricity. In the Morse system a "key" is used which, in its normal position, "breaks" the circuit, but when depressed by the linger of the operator allows the electricity to pass through it on its mission. Arrived at the distant station, it is converted, by means of an electro-magnet, into mechanical motion, which is utilized either to produce indentations in a moving slip of paper by means of a style, or, more commonly, to give a series of taps, which the operator understands, by an instrument called a "sounder."

In the automatic system the means employed are altogether different. The message is, first of all, prepared by punching holes in a narrow ribbon of paper. These perforations are so grouped as to represent the dots and dashes of the telegraphic alphabet, and by the punching-machine, which is very complicated, all that are required to form a letter are punched at one stroke. In comparing the two systems this must not be lost sight of, as the time taken in punching must, of course, be added to the time of transmission. The machine, however, does its work more quickly than the Morse operator with his key, and, the time occupied in transmitting being so vastly less, the "automatic" may claim to have rendered old-fashioned telegraphy comparatively slow.

After the perforated slip of paper has been prepared, it is taken to the operator's table, where it is made to move forward rapidly between a metallic drum and a needle carrying two small steel wheels which rest upon it. Drum and wheels form part of the circuit, which is broken by the non-conducting paper interposed and closed when the holes permit of the wheels and the metallic cylinder beneath coming into contact. At the receiving-station a very similar arrangement does duty as a register. The paper slip is there saturated with a certain chemical solution which renders its whole substance a good conductor, and, instead of the wheels, there is an iron style or "pen." When electricity arrives over the line, it decomposes the moisture of the paper into oxygen and hydrogen, and oxidizes or rusts the pen.



A little of this oxide is rubbed off by the quickly-moving paper, and enters into combination with the chemical still contained in it, producing a stain in the form of a dot or dash which corresponds with the holes punched in the paper at the sending-station. Where three holes come together, both wheels form a contact, and a dash is produced, because the second wheel touches the cylinder while the first passes over the paper between the upper holes.