25G

STANDARD MEASURES, BY WHITWORTH, BESSEL, &c.

[Ci.ass X.

meter; that to the right-hand is a combination of a screwabout ten threads to the inch, of a tangent screw andwheel with 400 teeth, and also a circle with 250 divisions.Therefore the divisions on the circle indicate T ^th of ^thof 3 J 5 th an inch, or of one millionth part of an inch.The micrometer placed at the left-hand is furnished witha screw with ten threads to one inch, and a circle dividedinto 500 parts, and thus one part of the circle correspondsto -^th part of an inch. On the upper side of the frame,and extending through its whole length, is placed a halfsquare groove made of steel, with its angle downwards,aud its upper edges horizontal. In this the measure isplaced. Between one end of the measure and the pointof the right-hand micrometer, a perfectly flat contact pieceof metal is placed, with its sides parallel; the other end ofthe measure abuts against the left-hand micrometer.

There are two methods of determining when contacttakes place, and hence the length of the standard measure.

1. By that which is termed the test of gravitation.

2. By that which is termed the galvanic test.

(1.) The test by gravitation is as follows:—

The experimenter moves the end of the screw of theright-hand micrometer through one millionth of an inch,by means of the right-hand micrometer; then carefullyraises the contact piece, and allows it to fall by its owngravity: lie then moves a screw through a second millionthof an inch, raises the contact piece as before, and so on,till the approach of the end of the micrometer screw tothe end of the measure prevents the contact piece fromdescending. This completes the measure.

(2.) The galvanic test is as follows:—

There is a small battery composed of a piece of zincand copper soldered together and immersed in rain water,without the admixture of any acid; this is connected withthe micrometer (which is insulated from the machine),and with a delicate galvanometer by means of coveredwires. The measure itself is also insulated from themachine.

By pursuing the same process as before, contact is indi-cated on completing the circuit by the deflection of theneedle of the galvanometer.

This beautiful and delicate apparatus seems to be capableof improvement: the ends of the measure exhibited areperfectly flat, and of its full size; the contact pieces aresomewhat smaller, but the constant and repeated contactmust wear the ends away, and soon alter its length. Fora standard bar, it would certainly be better to insert at itsends a smaller contact surface, of a substance much harderthan steel. A Council Medal was awarded to Messrs.Whitworth. (Awarded also in Class VI.)

Baumann* (Prussia, No. 7G, p. 1052,) exhibits Bessel’sstandard measure. It consists of a solid beam of maho-gany 4 feet f>^ inches in length (Prussian measure),7 inches 2 lines square. The two 3-feet bars to be com-pared are placed upon a carriage on five wheels, Minchrun on rails in a direction at right angles to the length ofthe bars. At each end of the carriage are receptacles forthe bars with screw adjustments for placing them in aproper position for comparison. At each end of the beamis placed a micrometer, consisting of a slider moved by ascrew having about 400 turns to the inch. On the sliderare two Ys, in -which rests a cylinder of steel about 7 linesin diameter, and 4^ inches long. The inner end (viz., theend directed toM-ards the middle of the beam) is groundconvex, the other end is a cone. The point of the conerests against a vertical plane of steel attached to the axisof a “ fuhlniveau ” (level contact) at a point about 0*25inches from the axis. The bars to be compared (bars ofsteel, not hardened, 0*75 inches square, having their endson rather a small circular disc at either end, which ismade of hardened steel, ground truly plane), are placed onthe carriage, with their axis about 1^ inches asunder.The carriage is moved till the axis of one of the barscoincides with the ends of the steel cylinders, and themicrometer screws are turned till the convex end of thesteel cylinders coming in contact with the end of the bar,the pointed end of each cylinder pressing against the steelplate attached to the axis of the level turns the level, tillthe bubble rests nearly in the middle. The divisions of

the head of the micrometer screws, and the divisions ofthe level scales at which the end of the bubbles rest, arethen read off. The slides are now withdrawn through asmall space, the carriage moved till the axes of the secondbar coincides with the axes of the steel cylinders, and themicrometer screws turned as before, till the convex endsof the cylinders come in contact with the ends of thesecond bar, the conical points turn the level till thebubbles rest nearly in the middle of the scale, the divisionsof the heads of the micrometer screws and divisions atwhich the ends of the bubbles of level rest are read oft’.This constitutes one comparison.

After a certain number of comparisons have been madein this manner, each bar is turned, so that the surfacewhich was undermost becomes uppermost, without turningeither of them end for end, and the same number of com-parisons made as in their original position. The object ofthis is to estimate any eiTor that might arise in makingthe ends of the bars coincide with the axes of the twosteel cylinders, supposing the plane end of the bars not tobe exactly at right angles to the axes. During the com-parisons the bars are covered by a wooden case, whichallows the heads of the micrometer screws and tubes ofthe levels to be seen, and has two openings, covered withglass for observing the thermometers placed on the barsto be compared.

The delicacy of this “ comparateur” is such that unlessextraordinary precautions be taken, the errors producedby the fluctuations of atmospheric temperature are muchgreater than the errors of measurement.

In order to eliminate error, arising from the heatradiating from the observer, Bessel recommends callingthe bars A and B; A should be brought between themicrometers, then B, B again, and then A. These fourmeasures he calls one comparison. The observer thenshould station himself on the other side of the com-parateur, and make a second comparison. Bessel made anumber of comparisons of two bars in a cellar, where thechange of temperature was very small.

The means of four comparisons, two before and twoafter, the bars were turned, differed from the mean offourteen sets of 1 inch from comparisons by the followingfractions of a line :—

- 0*00010- 0*00010

- 0*00003+ 0*00013

- 0*00001+ 0*00010+ 0*00002- 0*00002- 0*00011- 0*00018+ 0*00011+ 0*00009

0*00007

0-00000

To make the axes of the bars coincident with thecommon axis of the steel cylinders a ring carrying afiihlhebel (lever of contact) can be fastened on the end cfeither cylinder. Bring the fiihlhebel in contact with thatportion of the end of the bar which is cylindrical, andturn the cylinders round its axis. If the long arm of thefiihlhebel remains pointing to the same division of the arc,along which it moves, the end of The bar is strictlycentred, if not the long end of the lever will move on thegraduated arc.* A Prize Medal was awarded to Mr.Baumann.

Simms (No. 741, pp. 475-477,) exhibits the threestandard yards, prepared for Her Majesty's Commis-sioners, for the restoration of the standard of length, withtwo methods of supporting them; one by Professor Miller,consisting of a system of levers, by which an equal degreeof pressure is sustained upon eight equidistant points ofthe bar; the second by the Ilev. R. Sheepshanks, byfloating the bar in mercury, which is therefore equally

* For a complete description of this instrument, seeBessel's “ Darstellung dcr Entersucliungcn and Maass-regclin, &c.,” Berlin, 1839.