Fig. 40. Graduations on the Fractional-reading Micrometer
Micrometers may also be graduated as in Fig. 41. Instead of using the zero line on A as a base line, a point is taken one-fifth of a turn around A, and the graduated scale on B is placed to correspond, as shown in the engraving; also, instead of making lines a, b, etc., on A, full length, they are made about half an inch long, and the numerators are entirely omitted and the denominators placed at the end instead of under the line. To the ordinary user of the tool, this is all that is necessary for a perfectly clear reading of the fractions.[[20]]
Fig. 41. Another Method of Graduating for Fractional Reading
Sensitive Attachment for Measuring Instruments
No matter how finely and accurately micrometers and verniers may be made, dependence must in all cases be placed on the sensitiveness of a man's hand to obtain the exact dimensions of the piece to be measured. In order to overcome this difficulty and eliminate the personal equation in the manufacture of duplicate and interchangeable parts, the sensitive attachment to the micrometer shown in Fig. 42 may be used, and will be found of much value.
Fig. 42. Sensitive Micrometer Attachment
The auxiliary barrel A is held to the anvil of the micrometer by means of a thumb screw B. At the inside end of the barrel is a secondary anvil C, the base of which bears against the short arm of the indicating lever D. The action will be clearly seen by reference to the engraving. The micrometer is so set that when a gage, G, of exact size, is placed between the measuring points, the long arm of the indicator stands at the 0 mark. If the pieces being calipered vary in the least from the standard size it will be readily noted by the movement of the pointer. Hard rubber shapes turned from rough casting often vary from 0.003 to 0.005 inch after having passed the inspector's test with an ordinary micrometer. With this attachment the inspector's helper can detect very minute variations from the limit size. Anything within the limits of the micrometer can be made to show to the naked eye variations as small as a ten-thousandth inch.[[21]]