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.]

...[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 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

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],

David Thiel, “Fractional Calipers”

Edward R Kratfel and George R Drake,

Paul D. Q. Campbell,