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Effects of Expansion in Solids

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  P-260

2025-12-27

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The difference in temperature between the coldest winter days and the hottest days of summer is enough to make a perceptible change in the length of long pieces of metal. Telephone and telegraph wires sag more in summer than in winter. Suspension bridges, like the Brooklyn Bridge, are several inches higher in the middle in midwinter than in summer. Bridge work and steam boilers are put together with red-hot bolts, so that the parts may be more firmly held together when the bolts are cool.

The table shows that the coefficients of expansion of glass and platinum are nearly the same. It is for this reason, and because platinum does not oxidize, that this metal is used as the sealed-in wire in incandescent lamps. A substitute for platinum for this purpose can be made by using a compound wire, the core of which is a nickel-steel alloy, over which is a thin sheathing of copper. This compound wire has a coefficient of expansion slightly less than that of platinum. As it is less expensive, it is generally used in place of platinum.

 A thermostat is an example of the application of the unequal expansion of metals (Fig. 1). A compound bar of copper and iron is fixed at one end and free to move at the other. When the temperature rises the point P completes the circuit through the cell C and the bell B. When the temperature falls the circuit is made through B'. By choosing bells of a different tone, it is easy to tell whether a room, a greenhouse for example, is too hot or too cold. If electromagnets are substituted for the bells, they can be made to open and close the door that controls the draft of a furnace, so that the device automatically regulates the temperature.