. Physical laboratory experiments : mechanics, optics and heat. jaws too tightly. It isbest to turn the screw until the fingers just slip off the endof the barrel. THE SPHEROMETER. ^ Apparatus.—Spherometers are instruments designed tomeasure the thickness of small objects with a high degree ofprecision and to measure the radius of curvature of spherical »convex or concave surfaces. It is from this latter use thatthe instrument derives its name. * One form of this instrument (due to Perreaux) is shown in •Fig. 6. It consists essentially of a vertical micrometer-screw ,moving in a nut fixed at t
. Physical laboratory experiments : mechanics, optics and heat. jaws too tightly. It isbest to turn the screw until the fingers just slip off the endof the barrel. THE SPHEROMETER. ^ Apparatus.—Spherometers are instruments designed tomeasure the thickness of small objects with a high degree ofprecision and to measure the radius of curvature of spherical »convex or concave surfaces. It is from this latter use thatthe instrument derives its name. * One form of this instrument (due to Perreaux) is shown in •Fig. 6. It consists essentially of a vertical micrometer-screw ,moving in a nut fixed at the cen-tre of an equilateral tripod. The •head of the micrometer screw A •is a graduated circle, the position ofwhich is read on a fixed verticalscale B graduated in divisions equalto the pitch of the screw. One com- (plete revolution of the screw thus raises or lowers A through one divi-sion of the fixed scale. Fractions of ^a revolution are read off on the head $of the micrometer with reference tothis scale. # The sensitiveness of the instrument ». —• ^ » ,v,... > 18 PHYSICAL LABORATORY EXPERIMENTS depends on the precision with which one can detect when theend of the micrometer screw just makes proper contactwith the object placed beneath it. In the ordinary formwhere no special device is provided for facilitating thisobservation, the screw is turned until the instrument justturns as a whole on the end of the screw as a pivot, oruntil the hand can just detect a slight rocking motion ofthe mstrument like that of a table, one of whose legs istoo long. With these apparently somewhat crude methodsof detecting contact, settings to one or two thousandths ofa millimeter may readily be made. To increase the sensitiveness of the instrument, variousdevices have been invented. The Perreaux multiple leverattachment is that shown in the figure. The micrometerscrew is hollow and through it a steel rod passes freely. Theupper end of this rod acts upon a system of two multi
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