An
inch (plural:
inches; symbol
or abbreviation:
in or, sometimes, ″ – a
double prime) is the name of a
unit of
length in
a number of different systems, including
Imperial units, and
United States customary units.
There are 36 inches in a
yard and 12 inches in
a
foot. A corresponding unit of
area is the
square
inch and a corresponding unit of
volume
is the
cubic inch.
The inch is usually
the universal unit of measurement in the United States, and is widely used in the United Kingdom, and Canada, despite the
introduction of metric to the latter two in the 1960s and 1970s,
respectively. The inch is sometimes used informally in other
Commonwealth nations such as
Australia.
International inch
Effective
July 1, 1959, the United
States and countries of the British Commonwealth defined the
length of the international yard to be exactly
0.9144 meters. Consequently, the
international inch is defined to be equal to exactly 25.4
millimeters.
The international standard symbol for inch is
in
(see
ISO 31-1, Annex A). In some cases, the
inch is denoted by a
double prime,
which is often approximated by
double
quotes, and the
foot by a
prime, which is often approximated by an
apostrophe.
Equivalence to other units of length
Mid-19th century tool for converting between different standards of
the inch
1 international inch is equal to:
- 1,000 thou (1 thou is
0.001 inches.)
- 1,000 mil (1 mil = 1 thou =
0.001 inches)
- 1,000,000 microinches (1 μin is one millionth of an inch.)
- ~0.02778 yards (1 yard is equal to 36
inches.)
- 2.54 centimeters (1 centimeter is
equal to about 0.3937 international inches.)
Historical origin
The origin of the inch is disputed. Historically, in different
parts of the world (even different cities within the same country)
and at different points in time, the inch has referred to similar
but different standard lengths.
The English word
inch comes from Latin
uncia meaning "one twelfth part" (in this case,
one twelfth of a foot); the word
ounce (one twelfth of a troy pound) has the same
origin.
In some other languages, the word for "inch" is similar to or the
same as the word for "thumb"; for example, inch/thumb; inch/thumb;
inch,
pulgar thumb; inch,
polegar thumb; inch,
tumme thumb; inch,
tommel thumb; inch/thumb;
inch,
anguli finger; inch/thumb; inch/thumb.
Given the etymology of the word "inch", it would seem that the inch
is a unit derived from the
foot, but
this was probably only so in Latin and in Roman times. In English,
there are records of fairly precise definitions for the size of an
inch (whereas the definitions for the size of a foot are probably
anecdotal), so it seems that the foot was then defined as 12 times
this length. For example, the old English
ynche was
defined (by King David I of Scotland in about 1150) as the width of
an average man's thumb at the base of the nail, even including the
requirement to calculate the average of a small, a medium, and a
large man's measures. To account for the much larger length later
called an inch, there are also attempts to link it to the distance
between the tip of the thumb and the first joint of the thumb, but
this may be speculation.
There are records of the unit being used circa
AD 1000 (both
Laws of Æthelberht and
Laws of Ælfred).
An Anglo-Saxon unit of length was the
barleycorn. After 1066, 1 inch was equal to 3
barleycorn, which continued to be its legal definition for several
centuries, with the barleycorn being the base unit. One of the
earliest such definitions is that of 1324, where the legal
definition of the inch was set out in a statute of
Edward II of England, defining it as
"three grains of barley, dry and round, placed end to end,
lengthwise".
Similar definitions are recorded in both English and Welsh mediæval
law tracts. One, dating from the first half of the 10th century, is
contained in the Laws of Hywel Dda (see
Hywel
Dda) which superseded those of
Dyvnwal,
an even earlier definition of the inch in Wales. Both definitions,
as recorded in
Ancient Laws and Institutes of Wales (vol
i., pp. 184,187,189), are that "three lengths of a barleycorn is
the inch".
Charles
Butler, a mathematics teacher at Cheam School, in 1814 recorded the old legal definition of the
inch to be "three grains of sound ripe barley being taken out the
middle of the ear, well dried, and laid end to end in a row", and
placed the barleycorn, not the inch, as the base unit of the
English Long Measure system, from which all other units were
derived. John Bouvier similarly
recorded in his 1843 law dictionary that the barleycorn was the
fundamental measure. Butler observed, however, that "[a]s the
length of the barley-corn cannot be fixed, so the inch according to
this method will be uncertain", noting that a standard inch measure
was now (by his time) kept in the Exchequer chamber, Guildhall, and
that was the legal definition of the inch. This was a
point also made by George Long in his 1842 Penny Cyclopædia,
observing that standard measures had since surpassed the barleycorn
definition of the inch, and that to recover the inch measure from
its original definition, in the event that the standard measure
were destroyed, would involve the measurement of large numbers of
barleycorns and taking their average lengths. He noted that this
process would not perfectly recover the standard, since it might
introduce errors of anywhere between one hundredth and one tenth of
an inch in the definition of a yard.
One source says that the inch was at one time defined in terms of
the
yard, itself supposedly defined as the
distance between
Henry I of
England's nose and his thumb. This is unlikely as Henry was
born in 1068.
Prior to
the adoption of the international inch (see above), the United Kingdom and most countries of the British Commonwealth defined the
inch in terms of the Imperial Standard
Yard. But Canada had its own,
different, definition of the inch, defined in terms of metric units. The Canadian inch was
defined to be equal to 25.4
millimeters,
the amount later accepted as the international inch.
Metric or decimal inch
A
metric inch (25 mm instead of 25.4 mm)
was the equivalent of an inch under a former proposal for the
metrification and unification of the
English system of measures. It is now
considered to be a
strange unit of
measurement. See also "
metric
foot".
In
Sweden, between 1855 to 1863, the existing Swedish
"working inch" of ~24.74 mm was replaced by a "decimal inch"
of ~29.69 mm which was one tenth of the Swedish foot.
Proponents argued that a decimal system simplifies calculations.
However, having two different Swedish inch measures (and the
English inch on top of that) proved to be complicated. So in a
transition period between 1878 and 1889 the metric units were
introduced as the overall standard measures. However, the various
inches survived some time in building and construction
trades.
References
- Brian Lasater, The Dream of the West (Lulu.com, 2008),
p256
See also