isa-normaladvance-1909-00012

Description:	12THE NORMAL ADVANCEthe span the parts of this joint play in andout about a foot. A glass, when suddenlyheated, breaks on account of unequal expansion or if suddenly cooled breaks on accountof unequal contraction. Clocks and watcheslose time in summer, on account of lengthenedpendulum, or of increased friction of the parts.Thus, it is possible to use a clock as a thermometer.For convenience men have worked out theco-efficients of expansion for many differentsolids. For each solid, there are, of course,three co-efficients,—linear, superficial andcubical. The linear, superficial, or cubical coefficient of expansion is that fraction of abodys length, surface or volume, which it expands on being heated 1 degree. Suppose asquare plate of metal whose length is 1, to expand, on being heated 1 degree, by the quantity a then the side of the new square =l+a,and it area =(l+a)2=l+2a+a2. The a2 beinginsensible is dropped and leaves the area l+2a,a being the linear, and 2a the superficial coefficient of expansion. Hence to obtain thesuperficial co-efficient of expansion, multiplythe linear co-efficient by 2. If it is a cube thatis heated 1 degree, each side being 1+a, thevolume =(l+a)3=l+3a+3a2+a3. Here as formerly, the a2 and a3 may be neglected on account of their exceeding smallness. The volume of the expanded cube then is l+3a, 3a or 3times the linear co-efficient being the cubicalco-efficient of expansion.The co-efficients of several solids are given inthe following table:Linear Coef. Cubical Coef.Glass .000008 , .000024Platinum .0000088 .000026Iron .0000123 .000037Copper .000017 .000051Tin .000023 .000067Lead .000029 .000087Zinc .0000294 .000088Ice .00005 .00015There have been some remarkable instances,expansion in industrial life, which, thoughseemingly unexplicable at the time, when putunder the searchlight of scientific investigationhave not only increased our knowledge of thephysical world, but have added their influencein a practical way to the work of the world.For example in, 1851, the choir of BristolCathedral, England, was covered with sheetlead, the length of the covering being 60 feet,and its width 19 feet, 8 in. In 1853, it hadmoved bodily down through a distance of 18inches. The descent had been going on fromthe time the lead was laid down, and on attempt to stop it by driving nails into the rafters had failed, the force of descent being sufficient to draw out the nails. The roof was nota steep one—the lead was not sliding. Whatcaused its descent? This was the problem ofthe day. A scientist named Moseley solved it.The lead was exposed to the varying temperatures of the day and night. During the daythe heat caused it to expand. Had it lain upona horizontal surface, it would have expandedequally all round, but as it lay upon an inclined surface it expanded more freely downwards than upwards. Its motion was, therefore, that of a common earthworm,—it pushedits lower edge forward during the day, anddrew its upper edge after it during the night,and thus by degrees it crawled through a spaceof 18 inches in 2 years. Therefore today wedo not put lead roofing upon an inclined roof,having learned one of Natures little lessons.There is an invention called the Creepermade upon this principle by Stevenson, whichis used in physical laboratories. It is simplya flat bar of an expansible metal provided withequidistant rows of teeth along its upper andlower ends, and made to rest on an inclinedslab in which horizontal grooves are cut so asto offer the slightest resistance to motion upwards, but preventing all sliding down. Sothe bar gradually creeps up the incline,—anillustration of locomotion directly from heat,without the intervention of steam.Another remarkable instance occurred in oneof the smelting works of Saxony, a number ofyears ago. A quantity of silver, which hadbeen fused in a ladle was turned out upon an
Source:	http://indstate.contentdm.oclc.org/cdm/ref/collection/isuarchive/id/34042
Collection:	Indiana State University Archives

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