Much self-acting machinery employs electricity. By virtue of this wonderful agent the Automatic Electric Company of Chicago instals telephonic systems which enable a subscriber to connect himself directly with any other subscriber, without the intervention of an operator at the central station. As exemplified in large exchanges such as those of Dayton, Ohio, and Grand Rapids, Michigan, the apparatus is complex in its detail. If we take a small exchange, such as that of a village with 100 instruments, we may readily understand the main principles of the method. Let us suppose that No. 1 of our instruments is at the Post Office, where the Postmaster wishes to call 58. With a finger he moves hole 5 in the dial plate of his [calling instrument] (see the page opposite 336) until it touches a protruding stud. Then he lets go, when the dial returns to its original position. In returning it sends five impulses to the central office where a vertical rod is lifted five notches (see [illustration], page 336.) He next moves hole 8 to the stud and lets go. This time the rod turns through a considerable part of its semicircle of motion. The instant its journey is at an end a tiny metallic arm flies out and connection is completed with a wire running to 58, ringing his bell. In case he is busy, a buzzing noise will be heard in telephone No. 1. The switch mechanism which comes into play in all this is simple. There are ten rows of switches, ten in each row: the lowest row runs from 1 to 10, the next from 11 to 20, and so on. The upward motion of the vertical rod in our example brought it to the fifties; the turning motion decided that out of these fifties switch 58 should be connected with No. 1. When a conversation ends, hanging up the receiver sends a current over both wires of the circuit so as to release the selector rod, which returns to its original position.
If instead of a village we have a fairly large town, with an exchange of 1000 subscribers, a call for let us say 829 will involve taking to the stud first hole 8, then hole 2, and lastly hole 9. And so on for exchanges still larger. The pioneer inventor in automatic telephony was the late Mr. Almon B. Strowger.
Chemical Triggers.
From triggers electrical we now pass to triggers chemical. A gun may be charged with powder and remain for years perfectly at rest until a touch on the trigger explodes the powder with tremendous effect. The example is typical: nature and art abound with cases where a little energy, rightly directed, controls energy vastly, perhaps infinitely, greater in quantity. Often in a chemical compound the poise of attraction is so delicate that it may be disturbed by a breath, or by a note from a fiddle, as when either of these induces iodide of nitrogen to explode. A beam of light works the same result with a mixture of chlorine and hydrogen. One of the most familiar facts of chemistry is that a fuel, such as coal, may remain intact in air for ages. Once let a fragment of it be brought to flaming heat and all the rest of the mass will take fire too. Iron has a strong affinity for oxygen, but for union there must be at the beginning some moisture with the gas; the same is true of carbon. A burning jet of carbon monoxide may be extinguished by plunging it into a jar of dried oxygen. Gases from the throat of a blast furnace, at a temperature of 250° to 300° Centigrade, are not inflammable in the atmosphere until the air is moistened by steam or otherwise. Then in a flash combustion begins in earnest.
In photography we meet with similar facts: violet rays may begin an impression which yellow light can finish and finish only. Vulcanite is transparent to red and infra-red rays which, although without action upon an unexposed plate, are capable of continuing the action of actinic rays upon a plate which has been exposed for a very short time.
Why Weather is Uncertain.
From photography let us pass to a glance at the atmospheric conditions which greatly affect its work. The weather from day to day depends upon factors so variable and unstable that prediction beyond twenty-four hours is unsafe. “Suppose a stratum of air,” says Professor Balfour Stewart, “to be very nearly saturated with aqueous vapor; that is to say, to be just a little above the dew-point; while at the same time it is losing heat but slowly, so that if left to itself it would be a long time before moisture were deposited. Now such a stratum is in a very delicate state of molecular equilibrium, and the dropping into it of a small crystal of snow would at once cause a remarkable change. The snow would cool the air around it, and thus moisture would be deposited around the snowflake in the form of fine mist or dew. Now, this deposited mist or dew, being a liquid, and giving out all the rays of heat possible to its temperature, would send its heat into empty space much more rapidly than the saturated air; therefore it would become colder than the air around it. Thus more air would be cooled, and more mist or dew deposited; and so on until a complete change of condition should be brought about. In this imaginary case the tiniest possible flake of snow has pulled the trigger, as it were, and made the gun go off,—has altered completely the whole arrangement that might have gone on for some time longer as it was, had it not been for the advent of the snowflake. We thus see how in our atmosphere the presence of a condensable liquid adds an element of violence, and also of abruptness, amounting to incalculability, to the motions which take place. This means that our knowledge of meteorological phenomena can never be mathematically complete, like our knowledge of planetary motions, inasmuch as there exists an element of instability, and therefore of incalculability, in virtue of which a very considerable change may result from a very small cause.”
In view of the inherent difficulties it is certainly creditable that the predictions of the United States Weather Bureau should prove true six times in seven, greatly inuring to the safety of mariners, of passengers by lake and sea, and to the saving of crops under threat of destruction by storms.