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The Screw-Gauge

The screw gauge (fig. 3) consists of a piece of solid metal s, with two arms extending perpendicularly from its two ends. To the one arm a steel plug, P, with a carefully planed face, is fixed, and through the other arm, opposite to the plug, a screw C passes, having a plane face parallel and opposite to that of the plug. The pitch of the screw is half a millimetre, and consequently if we can count the number of turns and fractions of a turn of the screw from its position when the two plane faces (viz. that of the plug and that of the screw) are in contact, we can determine the distance in millimetres between these two parallel surfaces when the screw is in any position.

In order to do this the more conveniently, there is attached to the end of the screw farther from the plug a cap x, which slides over the cylindrical bar through which the screw passes; this cap has a bevelled edge, the circumference of which is divided into fifty equal parts. The circle on the cylindrical bar, which is immediately under the bevelled edge, when the two opposing plane surfaces are in contact, is marked L, and a line drawn parallel to the length of the cylinder is coincident (if the apparatus is in perfect adjustment) with one of the graduations on the bevelled edge; this we will call the zero line of that edge. Along this line a scale is graduated to half-millimetres, and hence one division of the scale corresponds to one complete turn of the cap and screw. Hence the distance between the parallel planes can be measured to half a millimetre by reading on this scale.

We require still to determine the fraction of a turn. We know that a complete revolution corresponds to half a millimetre; the rotating edge is divided into.fifty parts, and therefore a rotation through a single part corresponds to a separation of the parallel planes by 1/100 mm. Suppose, then, that the scale or line along which the graduations on the cylinder are marked, cuts the graduations on the edge of the cap at 12.2 divisions from the zero mark; then since, when a revolution is complete, the zero mark is coincident with the line along which the graduations are carried on the cylinder, the distance between the parallel planes exceeds the number of complete revolutions read on that scale by 12.2/50-ths of a turn, i.e. by 0.122 mm.

If then we number every tenth division on the bevelled edge successively 1, 2, 3, 4, 5, these numbers will indicate tenths of a millimetre; 5 of them will be a complete turn, and we must go into the next turn for 6, 7, 8, 9 tenths of a millimetre. It will be noticed that on the scale graduated on the fixed cylinder the smaller scratches correspond to the odd half-millimetres and the longer ones to the complete millimetres. And on the revolving edge there are two series of numbers, 1, 2, 3, 4, 5 inside, and 6, 7, 8, 9,10 outside. A little consideration will show that the number to be taken is the inside or the outside one according as the last visible division on the fixed scale is a complete millimetre division or an odd half-millimetre division.

We can therefore read by this instrument the distance between the parallel planes to 1/100th of a millimetre, or by estimating the tenth of a division on the rotating edge to the 1/1000th of a millimetre.

We may use the instrument to measure the length of a short cylinder thus. Turn the screw-cap, holding it quite lightly, so that, as soon as the two parallel planes touch, the fingers shall slip on the milled head, and accordingly shall not strain the screw by screwing too hard.(1) Take a reading when the two planes are in contact; this gives the zero reading, which must be added to any observation reading if the zero of the scale has been passed, subtracted if it has not been reached. Then separate the planes and introduce the cylinder with its ends parallel to those of the gauge, and screw up again, holding the screwhead as nearly as possible with the same grip as before, so that the ringers shall slip when the pressure is as before. Then read off on the scales. Add or subtract the zero correction as the case may be; a reading of the length of the cylinder is thus obtained. Read the zero again, and then the length of the cylinder at a different part of the area of the ends, and so on for ten readings, always correcting for the zero reading.

Take the mean of the readings for the length of the cylinder, and then determine the mean diameter in the same way.

The diameter of a wire may also conveniently be found by this instrument.

The success of the method depends on the touch of the screwhead, to make sure that the two planes are pressed together for the zero reading with the same pressure as when the cylinder is between them.

Be careful not to strain the screw by screwing too hard.

Experiment. - Measure the length and diameter of the given small cylinder.

Enter result thus : -

Correction for zero + 0.0003 cm
Length (mean of ten) 0.9957 cm
True length 0.9960 cm

(1) Special provision is made for this in an improved form of this apparatus. The milled head is arranged so that it slips past a ratchet wheel whenever the pressure on the screw-face exceeds a certain limit.

Last Update: 2011-03-27