88 PHYSICS. 
It is necessary, also, in certain circumstances, to anticipate by timely 
precautions acts that would arise from this property of bodies. Thus, if on 
a railroad the rails be laid in cold weather, with their ends in absolute 
contact, the summer heat will cause them to elongate, and, having no room 
to yield in length, to warp. The bars or rails must therefore be laid at the 
highest temperature, or with an interval sufficient for the greatest possible 
elongation. Similar cases occur in tubes for conducting steam, gas, or 
water, where it becomes necessary to employ special compensation pipes. 
The influence of temperature on pendulums and its compensation has already 
been referred to (p. 208), [Physics, 34]. Here belong the compensation 
bars, whose construction depends upon the fact, that different solids possess 
different expansibilities. If, for instance, two strips, one of zinc and the 
other of iron, be soldered together, forming a straight bar at a temperature 
of 20°R., then, at a temperature above this, the compound bar will become 
curved, and the zine will occupy the convexity of the curve; at a lower 
temperature the case will be reversed, the zinc now occupying the con- 
cavity. The cause of this lies in the fact, that at equal temperatures, zine 
both contracts and expands more than iron. 
Upon the arrangement of compensation strips depends the construction 
of quadrant or metal thermometers (pl. 19, fig. 12). The strip fgh, con- 
sisting of copper and steel, is attached at f, and curves at g towards h. 
Against it rests, at h, the short arm of a lever, movable in its axis, the 
longer arm, b, being provided with the rack, dd’. The latter catches in a 
pinion moving on the central axis, whose motion is magnified still more by 
the needle /i, turning on the same axis. With an increment of temperature 
the strip, fgh, becomes more curved, and the rack becomes turned in a 
direction from d towards d’, and with it likewise the needle serving as index. 
If the curvature be diminished by a bending in the opposite direction, a 
special spring wound about the axis produces a corresponding retrograde 
motion of the index. The compensation strip is so calculated, that the 
needle, at an increase of temperature of 80°R., shall make a complete 
revolution. The dial plate must be graduated separately for each instru- 
ment, by comparison with a good mercurial thermometer, and, if possible, 
degree by degree; as in the former the degrees are not equal, and cannot, 
therefore, as in the case of the latter, be described mechanically. 
The most sensitive metal thermometer is that of Breguet (p/. 19, fig. 13). 
It consists of a spirally wound compound band of metal, formed by soldering 
together three thin strips of silver, gold, and platinum. This is fastened at 
its upper extremity to a brass arm, the lower end being free. At this lower 
extremity is a very light horizontal needle, whose point traverses a scale on 
the upper edge of a ring, supported in three feet. For protection against 
external influences, the apparatus is covered by a glass bell. The needle 
is made to turn by the unequal expansion and contraction of the silver and 
platinum, with change of temperature; the use of the gold is merely to 
unite the two other metals. 
The expansion of liquids is not uniform at high temperatures, the most 
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