also Dial Caliper and Micrometer Caliper. Variations include Compass,Divider, and other types of calipers, many noted below. [need to supplement this entry with the other types of tools that fall roughly into the same category.]
See also Measurement.
A Caliper is
...[a] measuring instrument with a movable spindle for taking highly exact measurements.... Some such tools are capable of measuring a 10,000th of an inch.
(Anonymous article in 1935 Home Craftsman.)
Calipers are, according to Paul Hasluck (1903: 466),
... the tools used by the carpenter, but the turner uses them almost constantly for some jobs, whereas in ordinary carpentry they are used but seldom....
Further, Hasluck does not shy away from prescriptive comments:
For small callipers (sic) with which to measure up to a diameter of 2 114 in., the mild steel should be at least 314 in. wide and 1116 in. thick ; a length of 5 114 in. is sufficient for outside callipers, and 4 112 in. for inside ones. The washers may be 11/16 in, in diameter.
Called also "pair of calipers".
Dial Calipers appeared about three decades ago, an innovation "that made it easier to read the measurement", David Thiel (2006: 26) argues, but "still marked in decimals". (In my own experience, having a conversion calculator close by pays off.)
Early in the 21st century, though, to help woodworkers make measurements, but who are more familiar measuring fractions of inches, so-called Fractional Calipers are increasingly available on the market. (Rather than in the traditional one thousands, Fractional Calipers measure in inches.)
An outside caliper measures external dimensions; an inside caliper, internal dimensions. Calipers have many uses in the woodshop, like determining the thickness and/or diameter of Workpieces, or the distance between surfaces, such as width and/or depth of Dados, the diameter of a cylindrical hole, like a rounded Mortise.
As David Thiel notes, using calipers appeared in the 14th century, when the Great Wall of China was built, ca. 1368-1644.
(Adapted from the 1962 Columbia Encyclopedia: Approximately 1,500 miles long, the Great Wall of China winds across North China, from Kansu province, on the Yellow Sea to Chinwangtao, Hopeh province. Designed to protect China from northern barbarians, it amalgamates many walls built since ancient times along the southern edge of Mongolia. Averaging twenty-five feet in height, and fifteen to thick at its base, it slopes twelve at its top. The eastern part of the wall is earth and stone faced with brick, but in the west it is merely an earthen mound.)But the measuring units and the general design of the tool have changed over the centuries, with one of the largest improvements coming from the Frenchman, Pierre Vernier (1584-1683). Vernier -- a mapmaker who trained mathematician and scientist -- developed a caliper with two graduated scales that make it possible to take measurements accurate to a minute fraction of the division on the main scale. [Nice illustration needed here – the small diagram in Webster’s new dictionary 2d ed 1952 is perfect.] Vernier calipers -- most commonly marked in millimeters and decimal units -- are still in use today.
Its anatomy different than the Caliper described above, the Micrometer Caliper is a tool for precisely measuring diameters, thicknesses, and lengths of solid objects, like bolts, bearings, and metal turnings. It consists of a C-shaped frame with a movable jaw operated by a tubular screw. The accuracy of the measurements depends on the accuracy of the screw-nut combination. [illustration needed – Webster’s 2d, 1952, is still very useful.]
The Micrometer Caliper, a caliper with micrometer screw attached, is used for very exact measurement. notes in a definition, that a micrometer is "often made to measure to 0.0001”.
Developed by the seventeenth century French mathematician Pierre Vernier (1580-1637), the Vernier system uses two opposing sets of graduations with slightly different tick marks over a given span from one set to the opposing set.
This system may be placed with the two opposing sets of tick marks linear, on a set of circular barrels, or on opposing linear arcs for angular Verniers. Linear Vernier scales are typically employed on an instrument called a "caliper," barrel-type Verniers are used on "micrometers," and linear arc Verniers are employed on specialized protractors. Here we will start with an explanation of how to read and use a linear Vernier as a preface to covering the use of the caliper in the next section. This will simplify the explanation of the barrel-type Vernier used on the micrometer when we get to that section. Fig. 2-6 illustrates a typical linear Vernier scale. Note that there are two sets of numbers: one on the "bar," which is a fixed part of the instrument, and the other set on the sliding jaw. The increments, shown here in English units, are divided in tenths, but notice the smallest subdivisions are not fiftieths, rather they are fortieths, or 0.025 of an inch. However, the opposing scale of numbers actually makes this instrument accurate to 0.001 inch.
The incremental tick marks on the sliding scale are spaced such that the 25 graduations cover exactly the same span as 24 graduations of the bar's scale. In other words, each of the graduations on the sliding scale is made 1/25 smaller than the bar scale graduations to allow for one more. If the bar scale graduations are 1/40 of an inch, and the sliding graduations are 1/25 smaller, then 1/25 of 1/40 just happens to equal 1/1000.
Reading the Vernier is done initially from the zero line of the sliding scale. For example, in the illustration, all of the graduations to the left of the zero line of the sliding scale are first added to the distance. First, this means it has passed the one-inch mark; then we count the tenth increments, which in this case are three, so we now have 1.300. Next, each of the smallest tick marks to the left of the sliding scale zero line is added, in this case, there are two ticks after the three-tenths mark still to the left of the zero line. We have already determined that each of these tick marks represents 0.025 inch, therefore, we now add 0.050 to our initial result, i.e., 1.300 + 0.050 = 1.350.
Sources:
Paul Noonan Hasluck, The
Handyman's Book, London: Cassell, 1903;
[Anonymous], Home Craftsman 4
1935 July August p 260;
David Thiel, “Fractional
Calipers”
Woodworking Magazine
Spring 2006, pages 26-29;
Columbia Encyclopedia,1962;
Oxford
English Dictionary;
Webster's New
Dictionary 2d ed;
Edward R Kratfel and George R Drake, Modern Shop Procedures Reston Virginia, 1974, pages 16-26;
Paul D. Q. Campbell, An
Introduction to Measuration and Calibration,New York: Industrial Press, 1995.