Fig. 360.—Bessel compensated rod.
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758.—Compensated Rods.—The plan used by Bessel for the measurement of a base upon the shores of the Baltic in 1836 is looked upon as a model of the most perfect work of its kind. The rods were composed of two bars of iron having surfaces accurately planed, with a similar bar of zinc placed between them. The bars were laid one on the other, but not in contact, the surfaces being kept apart by glass plates, upon which they could slide with little friction. The linear expansion of zinc per degree centigrade is about ·0000292 (Fizeau); that of iron much less than half this—about ·0000119 (Thomson). The bars are attached to each other in such a way that the expansion of the zinc may act in the opposite direction to the expansion of the iron. The form followed for the construction is shown in Fig. 360 where II′ are the iron bars, Z zinc. The length of the zinc required for compensation between the junctions is found in the equation—
(S + Z)(0000119) - Z(0000292) = 0,
S being the total length of the standard rod in feet, and Z the length of zinc in feet required for compensation. This plan is that adopted for the compensation of pendulums. For the verification of a rod it may also be made to form the rod of a pendulum, by which temperature expansion and contraction upon the system will be clearly indicated by difference of time rate in the change of temperature during night and day. This test becomes important where great precision is aimed at, as the expansion in metals varies according to their purity and state. The standard lines in rods made upon this model are placed upon small inserted discs of platinum placed near the ends, which are read by microscopes in coincidence upon a pair of rods.
759.—Colby Compensated Rods, the invention of Major-General T. F. Colby, who was for twenty-seven years superintendent of the Ordnance Survey, upon which these rods were used. Each rod is composed of one rod of iron and one of brass, which are so arranged in pairs that the difference of expansion of these metals shall act to diminish the amount of entire expansion at the points measured, a quantity equal to its increase by temperature, in a manner to be described.
The Rods are each made 10 feet 1·5 inches long, 5 inches broad, and 1·5 inches deep. Fig. 362 i is a side elevation of one rod, Fig. 363 ib plan of iron and brass rods, Fig. 365 ib perspective view. By this it will be seen that the rods are placed edgewise. The distance apart is 1·125 inches. They are supported in the middle upon rollers, Fig. 362 F. They are firmly fixed together at their centres by transverse steel cylinders, Fig. 363 RR′ 1·5 inches diameter, each rod being left free to expand or contract from the neutral central point independently of the other. The neutral point is formed of a T-piece E, Fig. 363, fixed firmly on the bottom of the box bx. At the extremity, and at right angles to each of these bars, is a flat steel tongue, Figs. 364, 365 A, 6·2 inches long, 1·1 inches broad, and 0·25 inch thick, which projects 3·25 inches from the side of the iron bar i. The tongue A is jointed by double conical pivots at f and f′, which form axes perpendicular to the surface of the tongue, allowing it to be inclined to slightly different angles to the direction of the bars according to the expansion or contraction the system experiences by heat. The pivots are 0·5 inch diameter, and are placed at 2·3 inches from the end of the tongue next the brass bar. On the tongue at P, flush with its upper surface, a small stud of platinum is inserted, upon which a small dot is struck to form the point of standard measurement.
Colby Compensated Measuring Rods.