Man's physical universe

xanabras

224 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME

Allowances must he made for expansion and contraction in steam

pipes and water pipes in buildings, in long pipe lines, in the construction

of bridges, paving, and railroad tracks. Expansion in pipes is permitted

by the use of tight sleeves, within which the pipes can work

back and forth. Pipe lines use large loops, whose change in curvature

will take care of expansion and contraction.

Rivets in steel girders and sheets are put in place while hot, partly

because they are softer and thus easier to work at high temperatures,

but also because they contract on cooling to form very tight joints.

Cracks are left between sections in concrete paving; these cracks are

generally filled with tar.

On a very hot day the tar may sometimes be

seen to bulge up at the cracks due to the expansion of the concrete.

Bimetallic-strip thermostats, widely used in temperature-regulating

devices, consist of two strips of metal with different coefificients of

expansion, such as brass and steel, fastened together so that the strip

is bent when it is heated. Some thermometers built into ovens work

on this principle.

Thermostats of certain types utilize bimetallic strips whose motion

closes electric contacts or air-pressure lines, which thus regulate the

heat supply or the electricity for house-heating systems, hot-water

heaters, refrigerators, and electric irons.

The coefficients of linear expansion of a few common substances are

shown in the following table:

Material

Ratio of Increase in Length to

Length at 0° C, per Degree C.

Aluminum 0.0000255

Concrete

0.0000168 (varies with composition)

Copper 0.000014

Steel 0.000013 (varies with composition)

Lime 0.000009 (varies with composition)

glass (ordinary) . .

Platinum 0.000009

Borosilicate glass .... 0.000003 (varies with composition)

Ordinary glass has to be annealed after it has been heated to a high

temperature. Annealing is accomplished by cooling the glass slowly so

that one portion will not cool more rapidly than another, thus avoiding

strains in the glass which would result in the glass breaking too easily.

Borosilicate glass has a much lower coefficient of expansion than

ordinary glass and may therefore be heated to high temperatures and

be cooled quickly without fracture. For that reason it has been used

in making oven-ware and laboratory apparatus.

Utensils now made of

specially treated low-expansion glasses may be used for cooking foods

over a free flame.

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