Fig. 1392.
Fig. 1393.
[Fig. 1392] represents a vernier caliper, in which the measurement is read by the coincidence of ruled lines upon the following principle. The vernier is a device for subdividing the readings of any equidistant lines of division. Its principle of action may be explained as follows: Suppose in [Fig. 1393] a to be a rule or scale divided into inches and tenths of an inch, and b a vernier so divided that its ten equidistant divisions are equal to nine of the divisions on a; then the distance apart of the lines of division on a will be 1⁄10 inch; but, as the whole ten divisions on b measure less than an inch, by 1⁄10 inch, then each line of division is a tenth part of the lacking tenth less than 1⁄10 inch apart. Thus, were we to take a space equal to the 1⁄10 inch between 9 and 10 on a, and divide it into 10 equal parts (which would give ten parts each measuring 1⁄100th of an inch) and add one of said parts to each of the distances between the lines of division on b, then the whole of the lines on a would coincide with those on b. It becomes evident, then, that line 1 on b is 1⁄100 inch below line 1 on a, that line 2 on b is 2⁄100 inch below line 2 on a, line 3 on the vernier b is 3⁄100 inch below line 3 on the rule a, and so on, until we arrive at line 10 on the vernier, which is 10⁄100 or 1⁄10 inch below line 10 on a. Suppose, then, the rule or scale to rest vertically on a truly surfaced plate, and a piece of metal be placed beneath b, the thickness of the piece will be shown by which of the lines on b coincides with a line on a. For more minute divisions it is simply necessary to have more lines of division in a given length on a and b. Thus, if the rule be divided into inches and fiftieths, and the vernier is so divided that it has 20 equidistant lines of division to 19 lines on the rule, it will then lack one division, or 1⁄50 inch in 20⁄50 inch, each division on the vernier will then be the one-twentieth of a fiftieth too short, and as 1⁄20 of 1⁄50 is 1⁄1000, the instrument will read to one-thousandth of an inch.
Let it now be noted that, instead of making the lines of division closer together to obtain minute measurements, the same end may be obtained by making the vernier longer. For example, suppose it be required to measure to 1⁄2000 part of an inch, then, if the rule or scale be graduated to inches and fiftieths, and the vernier be graduated to have 40 equidistant lines of division, and 39 of the lines on the scale, the reading will be to the 1⁄2000 part of an inch. But, in any event, the whole of the readings on the vernier may be read, or will be passed through, while it is traversing a division equal to one of the divisions on the scale or rule.
In [Fig. 1392] is shown a vernier caliper, in which the vernier is attached to and carried by a slide operating against the inside edge of the instrument. The bar is marked or graduated on one side by lines showing inches and fiftieths of an inch, with a vernier graduated to have 20 equidistant lines of division in 19 of the lines of division on the bar, and therefore measuring to the 1⁄1000th of an inch, while the other side is marked in millimètres with a vernier reading to 1⁄40th millimètre, there being also 20 lines of division on the vernier to 19 on the bar.
The inside surfaces of the feet or jaws are relieved from the bar to about the middle of their lengths, so as to confine the measuring surfaces to dimensions sufficiently small to insure accurate measurement, while large enough to provide a bearing area not subject to rapid wear. If the jaw surface had contact from the point to the bar, it would be impossible to employ the instrument upon a rectangular having a burr, or slight projection, on the edge. Again, by confining the bearing area to as small limits as consistent with the requirements of durability a smaller area of the measured work is covered, and the undulations of the same may be more minutely followed.
To maintain the surface of the movable jaw parallel with that of the bar-jaw, it is necessary that the edge of the slide carrying the vernier be maintained in proper contact with the edge of the instrument, which, while adjusting the vernier, should be accomplished as follows:—