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SPHEROMETER (Gr. ocPaipa , a sphere, ...

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Originally appearing in Volume V25, Page 661 of the 1911 Encyclopedia Britannica.
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SPHEROMETER (Gr. ocPaipa , a See also:sphere, p rpov, a measure) , an See also:instrument for the precise measurement of the See also:radius of a sphere or the thickness of a thin See also:plate. The usual See also:form consists of a See also:fine See also:screw moving in a See also:nut carried on the centre of a small three-legged table; the feet forming the vertices of an equilateral triangle (see figure). The See also:lower end of the screw and those of the table legs are finely tapered and terminate in hemispheres, so that each rests on a point. If the screw has two turns of the See also:thread to the millimetre the See also:head is usually divided into 50o equal parts, so that See also:differences of o•oot millimetre may be measured without using a See also:vernier. A See also:lens, however, may be fitted, in See also:order to magnify the. See also:scale divisions. A See also:vertical scale fastened to the table indicates the number of whole turns of the screw and serves as an See also:index for See also:reading the divisions on the head. In order to measure the thickness of a plate the instrument is placed on a perfectly level See also:plane See also:surface and the screw turned until the point just touches; the' exact instant when it does so is defined by a sudden diminution of resistance succeeded by a considerable in-crease. The divided head and scale are read ; the screw is raised ; the thin plate slipped under it; and the See also:process is repeated. The difference between the two readings gives the required thickness. A contact- See also:lever, delicate level or electric contact arrangement may be attached to the spherometer in order to indicate the moment of touching more precisely than is possible by the sense of See also:touch. To measure the radius of a sphere—e.g. the curvature of a lens-the spherometer is levelled and read, then placed on the sphere, adjusted until the four points exert equal pressure, and read again. The difference gives the thickness of that portion of the sphere cut off by a plane passing through the three feet.

Calling this distance h, and the distance between the feet a, the radius R is given by the See also:

formula R = (See also:a2+3h2)/6h.

End of Article: SPHEROMETER (Gr. ocPaipa , a sphere, p rpov, a measure)

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