Delicate Measurement.

Delicate Measurement.

There are plenty of men who will, by the impression carried through a pair of calipers and the finger’s ends, determine, within a very small percentage, the amount of pressure which shall be required to be exerted by a hydraulic press in order to force onto its shaft an engine crank or a locomotive drivingwheel ; a measurement in which a thousandth part of an inch variation in diameter causes much more variation in pressure than is permissible. Indeed, on some kinds of work done in the machine shops a thousandth part of an inch has now become the most commonly employed unit of measurement ; a unit which is divided and subdivided into at least ten parts in order to express the degree of refinement arrived at. This, of course, far surpasses the frequently mentioned but supposedly superfluous hair-splitting operations, since an ordinary human hair is about two and one-half thousandths of an inch in diameter. The paper upon which this page is printed is about three thousandths of an inch thick, and one ten-thousandth part of an inch is, therefore, one-thirtieth the thickness of this sheet. Considerably smaller variations of size can be detected by the trained sense of touch, or rather by the variation in the resistance of a pair of calipers passed over the work, and it is even possible for the sense of magnitude, and the sensitiveness of the finger ends in relation to it, to be so highly developed as to detect, unaided, and by merely rolling a small steel ball between the thumb and finger, a variation from true sphericity amounting to 1-125000 of an inch, cr about one thirty-seventh part of the thickness of the paper of this page ; Ambrose Webster, a machinist of Waltham, whose business is the making of machinery and tools for watch manufacture, having demonstrated his ability to do this.

There are plenty of men who will, by the impression carried through a pair of calipers and the finger’s ends, determine, within a very small percentage, the amount of pressure which shall be required to be exerted by a hydraulic press in order to force onto its shaft an engine crank or a locomotive drivingwheel ; a measurement in which a thousandth part of an inch variation in diameter causes much more variation in pressure than is permissible. Indeed, on some kinds of work done in the machine shops a thousandth part of an inch has now become the most commonly employed unit of measurement ; a unit which is divided and subdivided into at least ten parts in order to express the degree of refinement arrived at. This, of course, far surpasses the frequently mentioned but supposedly superfluous hair-splitting operations, since an ordinary human hair is about two and one-half thousandths of an inch in diameter. The paper upon which this page is printed is about three thousandths of an inch thick, and one ten-thousandth part of an inch is, therefore, one-thirtieth the thickness of this sheet. Considerably smaller variations of size can be detected by the trained sense of touch, or rather by the variation in the resistance of a pair of calipers passed over the work, and it is even possible for the sense of magnitude, and the sensitiveness of the finger ends in relation to it, to be so highly developed as to detect, unaided, and by merely rolling a small steel ball between the thumb and finger, a variation from true sphericity amounting to 1-125000 of an inch, cr about one thirty-seventh part of the thickness of the paper of this page ; Ambrose Webster, a machinist of Waltham, whose business is the making of machinery and tools for watch manufacture, having demonstrated his ability to do this.