Understanding Physics, 3 Volumes in 1 - Motion, Sound, & Heat; Light, Magnetism, & Electricity; The Electron, Proton & Neutron by Isaac Asimov

Author:Isaac Asimov [Asimov, Isaac]
Language: eng
Format: epub
Amazon: B000RG7YPG
Publisher: Barnes & Noble
Published: 1993-11-15T00:00:00+00:00

184 Understanding Physics length) to nearly twice the extent that an iron strip of the same size will. If the two strips of metal were not welded together, the brass would expand under the influence of increasing temperature and slide against the iron, becoming a trifle the longer of the two although they had been equal in length to start with. If the temperature is reduced below the starting point, matters are reversed. Now the brass contracts more than the iron does, sliding against the iron and ending a trifle the shorter of the two. However, the two strips of metal are welded together and the brass cannot slide against the iron. What happens then is that the welded strips (a bimetallic strip or a compound bar) bend in the direction of the iron if it is heated. The brass would then lie along the outer rim of the curve and the iron along the inner. Since the outer rim is longer than the inner, this allows the brass to be longer than the iron while remaining welded throughout. As the temperature falls again, the curve straightens and becomes entirely straight when the temperature returns to its original value. If the temperature falls lower still, the bimetallic strip bends in the direction of the brass, which now lies on the inner rim while the iron lies on the outer rim. If such a bimetallic strip is fixed at one end, the other end sways back and forth as temperature changes. The outer rim of the curved strip is very little longer than the inner, and the difference in length increases but slowly with the degree of bending. For that reason, even small changes in temperature, producing very small differences in length between the iron and brass, nevertheless produce a considerable amount of bending. A device of this sort can be used as a thermostat. As the temperature in the house falls, the bimetallic strip begins to curve to the left, let us say, and at a certain temperature the bending is sufficient for it to close an electrical contact that turns on the furnace. As the house heats up, the bimetallic strip bends back and quickly breaks the contact, thereby turning the furnace off. By altering the position of the electrical contact (easily done by hand), we can arrange to have the bimetallic strip turn the furnace on at any temperature we please. Then, too, the exact position of the free end of such a bi-, metallic strip can be used as a measure of the temperature. If a pen is attached and a circle of paper is allowed to revolve under it at a fixed speed, the device will automatically make a continuous recording of its position, and temperatures can be deduced from the position of the wavering line. Again, penduiums^xan be designed in which the rod is not

Temperature 185 a single piece of metal but several strips of two different metals— say, steel and zinc. These can be

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