Page:Popular Science Monthly Volume 9.djvu/91

Rh on each side of W, it will not move that wheel, but it will move the wheel X by destroying the balance which previously existed there. But, if a similar extra volume be at the same time sent from B, the pressure in that part of the circuit between W and X will overcome the opposing forces at each of the points, and both wheels will be worked, each virtually by the distant reservoir and not by its own.

If we substitute galvanic batteries for the reservoirs, wires for the water-courses, and electricity for the water, this gives us the principle of the duplex telegraph, and it is obvious that no currents passing one another in contrary directions are necessary to it. It will be well to keep this in mind when we come to describe the quadruplex system.

Following the duplex, the American Automatic system may be said to have been perfected in 1873. The great rapidity with which messages are transmitted and recorded by it is its principal advantage, but it has others—as requiring a smaller force of operators and less specially skilled. The usual work of a Morse operator is acknowledged to be about 1,500 words an hour, and European operators do not average half as much; but, by the automatic method, to receive and print double that number of words per minute is an ordinary feat, and as many as 7,000 words—fourteen pages of this magazine—have been legibly recorded in that time. As every word contains, on an average, five letters, and as each letter is represented by a varying



number of dots and dashes, each formed by a separate discharge, the circuit, it is calculated, must be "closed" and "broken," and the chemicals in the battery must cease and recommence their action 60,000 times per minute, in the ordinary working of the automatic system.