Electrical art teems with rules that work both ways. Oersted observed that a current traversing a wire deflects a nearby compass needle. Faraday, with the guiding law of reciprocity ever in mind, forcibly deflected a magnetic needle so as to create a current in a neighboring wire by the motion of his hand. He thus discovered magneto-electricity, in Tyndall’s opinion the greatest result ever obtained by an experiment. On the simple principle then discovered by Faraday are built the huge generators that revolve at Niagara, at power-houses large and small throughout the world, for the production of electricity by mechanical motion. A compass needle has a field, or breadth of influence, surrounding its surface, which is small and weak. A monster magnet in a generator has a field at once large and strong. When an electrical conductor, such as a coil of copper wire, is forcibly rotated in that field, powerful currents of electricity arise in the wire, equivalent as energy to the mechanical effort of rotation. Take another case: a current decomposes water; the resulting gases as they combine yield just such a current as that which parted them. Join a strip of bismuth to a strip of antimony, and let a current traverse the pair; the junction will become heated. At another time, using no current, touch that joint with the hand for a moment; the communicated warmth, though trifling in amount, creates a current plainly revealed by a galvanometer, affording a delicate means of detecting minute changes of temperature. In 1874 M. Gramme showed four of his dynamos at the Vienna Exhibition. M. Fontaine, an electrician, saw a pair of loose wires near one of the machines and attached them to its terminals; the other ends of the wires happened to be connected with a dynamo in swift rotation. Immediately the newly attached machine began to revolve in a reverse direction as a motor. Thus by an accident, wisely followed up, did electricity add itself to motive powers, establishing an industry now of commanding importance.
In the chemical effects of a current we have parallel facts. Expose a nickel-iron plate to the alkaline bath of an Edison storage cell; at once the metal begins to dissolve, yielding a current. Now send a slightly stronger current into that plate; forthwith the plate picks out iron-nickel from its compounds in the liquid, growing fast to its original bulk. So many cases of this kind occur that chemists believe that synthesis and electrolysis are always counterparts. Be that as it may, we must remember that often chemical action is much more intricate than it seems to be at first sight. Thus in dry air, or even in dry oxygen, iron is unattacked; but bring in a little moisture and at once oxidation proceeds with rapid pace. So with the combustible gases emerging from the throat of a blast furnace; they refuse to burn until they meet a whiff of steam, when they instantly burst into flame. Chemical energy usually moves in a labyrinth which the chemist may be able to thread only in one direction. A retracing of his steps is for the day when he will know much more than he does now.
Ovens and Safes.
Properties purely physical, and therefore much simpler than those studied by the chemist, offer us noteworthy instances of rules that work both ways. For years the walls and doors of safes and bank vaults have been filled with gypsum as a substance all but impervious to heat. To-day Norwegian cooking chests, on much the same principle, are attracting public attention by their economy. A pot is filled with, let us say, the materials for soup, it is brought to a boil, and then placed in a chest thickly clad with a non-conducting coat of felt or even of hay, as illustrated on page 189. In an hour or so a capital soup is found to have cooked itself simply by its own retained heat. A resource long familiar to the builder of safes and strong-boxes is thus taken into household service with much profit. It is plain that whatever obstructs the passing of heat may be employed either to keep it in or keep it out. For years inventors busied themselves in finding non-conductors wherewith to cover steam-pipes and steam-boilers. To-day, in cold storage plants, these non-conductors are just as useful in covering pipes filled with circulating liquids of freezing temperatures. Take a parallel case in the field of physical research. In 1873 Dulong and Petit in their measurement of heat avoided losses of heat with a new approach to perfection by using glass vessels one inside another, with exhausted spaces in between. In 1892 Professor Dewar applied this device to keeping liquefied gases, of extremely low temperatures, from being warmed by surrounding bodies, an aim just the converse of that of Dulong and Petit. Often, as in these cases, the applications of a quality may come in pairs; one invention may suggest its twin.
Copyright, Pach Bros., New York.
THOMAS ALVA EDISON, 1906.
Orange, New Jersey.
This convertibility of principle may be observed as clearly in the phenomena of nature as in the creations of ingenuity. Water expands as it freezes; when this expansion takes place freely, the freezing temperature is 0° C., but when expansion is resisted, as when the water is confined in a strong gun-barrel, the freezing temperature is lowered, for now the ice has to do work in the act of crystallization. So with the boiling points of liquids; they rise as atmospheric pressure increases, they fall as atmospheric pressure is reduced. A prospector on Pike’s Peak cannot boil an egg in his kettle. Next day he descends a mine in the valley, to find the boiling point higher than when he built his fire beside the mouth of the mine.
Cube Root Easily Found.
Take another example of inversion, this time in the field of mensuration. Every schoolboy knows that cubes respectively one, two, three, and four inches in diameter have contents respectively of one, eight, twenty-seven, and sixty-four cubic inches; that is, the contents vary as the cubes of the diameters of these solids. This is true of all solids alike in form. Cones, therefore, which have an angle of let us say fifteen degrees at the apex, vary in contents as the cube of their heights. Cones usually are looked at as they rest on their bases; it is worth while to consider them reversed, pointing downward. An inverted cone, duly supported on a frame allowing motion upward and downward, and dipping into a cylinder partly filled with water, is a simple means of extracting cube root within say one and ten as limits. The cone should be marked off into tenths, and the cylinder, between high and low-water, into thousandths. On a similar plan a tapering wedge acts as a square-root extractor, displacing water as the square of its depth of immersion.