- a traditional British name for a small load,
especially a small load of hay. Never standardized, the jag represented roughly
20-25 bushels (0.7-1.0 cubic meters).
- jansky (Jy)
- a unit used in radio astronomy to measure the strength, or more precisely
the flux density, of radio signals from space. In measuring signal strength,
it's necessary to take into account both the area of the receiving antenna
and the width of the frequency band in which the signal occurs. Accordingly,
one jansky equals a flux of 10-26 watts
per square meter of receiving area per hertz
of frequency band (W/m2Hz). Although it is not an SI
unit, the jansky is approved by the International Astronomical Union and is
widely used by astronomers. It honors Karl G. Jansky (1905-1950), the American
electrical engineer who discovered radio waves from space in 1930. The jansky
is sometimes called the flux unit.
- a traditional unit of electric capacitance, approximately equal to the capacitance
of one of the Leiden jars used in electrical experiments as long ago as the
eighteenth century. Benjamin Franklin is said to have measured the storage
power of his electrical equipment in jars. There are 9 x 108 jars
in a farad, so 1 jar is approximately 1.1 nanofarad.
- jerib or djerib
- a traditional unit of land area in the Middle East and southwestern Asia.
The jerib originally varied considerably from one area to another. In modern
times it has become identified with the hectare
in many countries, including Turkey and Iran. In Afghanistan, however, it
is usually equal to 1/5 hectare (2000 square meters or 0.494 acre).
- a name sometimes used by engineers for the rate of change in the acceleration
of an object. This is not as silly as it might sound, because when we're in
a vehicle and feel a jerk, we are in fact experiencing a change in the acceleration
of the vehicle. The term has also been used as a unit of rate of change in
acceleration equal to a change in acceleration of one foot per second per
second in one second, that is, 1 ft/sec3. In this usage, one jerk
equals 0.3048 m/s3 or about 0.03108g
- a large wine bottle holding about 3 liters, 4 times the volume of a regular
bottle. A jeroboam is sometimes called a double
- Jersey foot
- a traditional unit of distance in Jersey, one of the Channel Islands.
Also called the pied-perche, the Jersey foot is equal to 11 English
inches or 0.9167 English foot.
The Jersey foot is divided into 12 Jersey inches, making 13.09 Jersey
inches to an English foot.
- jiffy 
- a unit of time used in computer engineering. A jiffy is the length of one
cycle, or tick, of the computer's system clock. In the past, this was
often equal to one period of the alternating current powering the computer:
1/60 second in the U.S. and Canada, usually 1/50 second elsewhere. More recently
the jiffy has become standardized, more or less, as 0.01 second (10 milliseconds).
The word jiffy, with its ordinary meaning of an instant or very brief time,
appeared in English during the eighteenth century, but its origin is not known.
- jiffy 
- a unit of time used in chemistry and physics, equal to a "light
that is, the time required for light to travel a distance of one centimeter.
This is a very brief interval indeed, about 33.3564 picoseconds. This definition
of the jiffy was proposed by the American physical chemist Gilbert N. Lewis
(1875-1946), who was one of the first to apply principles of quantum physics
in chemistry. More recently, some physicists have defined the jiffy as the
time required for light to travel a distance of one femtometer (fermi);
this would make the jiffy equal to 3.335 64 zeptoseconds.
- a unit of volume for liquor, usually considered equal to 1.5 (U.S.) fluid
ounces or 44.360 milliliters.
- a traditional unit of weight in China, comparable to the English pound.
During the European colonial era the jin was identified with the catty,
a Malay unit widely used in various forms throughout East and Southeast
Asia. Like the catty, the jin was then equal to 1 1/3 pounds or 604.79
grams. Traditionally, it was divided into 16 liang.
In modern China, however, the jin is a metric unit equal to exactly 500
grams (1.1023 pounds) and divided into 10 liang. The kilogram itself is
usually called the gongjin,
or "metric jin" (see gong-). The spellings chin and gin also
have been used for the jin.
- a traditional unit of land area in the Czech Republic, identical to the
Austrian joch (see below).
- an informal unit of area used in Japan to measure the size of rooms in houses
and apartments. One jo is the area of a traditional tatami mat, 180 by 90
centimeters or 1.62 square meters (1.94 square yards).
- a traditional unit of area in German speaking countries, especially in Austria.
One joch is the area of a square 40 klafters (about 83 yards) on a side. This
comes to 0.5755 hectare or about 1.422 acres.
The plural is joche. Joch is also the word for a yoke in German, so
this unit represents an area that could be plowed in a day by a yoke of oxen.
In what is now the Czech Republic this unit was known as the jitro;
in Croatia it is the jutro.
- a unit of detectivity, that is, the ability of an electronic device to detect
radiant energy such as light waves or infrared radiation. In a 1959 paper,
R. Clark Jones defined the "specific detectivity" of a device to be D*
= [square root (Aw)]/N, where A is the area of
the detector, w is the frequency bandwidth, and N is the power
of the noise generated by the device. The quantity D* is measured in
the complex unit cm·Hz1/2/W, customarily called the "Jones."
In modern equipment, detectivities are often quite large, in the range of
109 to 1012 Jones.
- joule (J)
- the SI unit of work or energy, defined to be the
work done by a force of one newton acting
to move an object through a distance of one meter in the direction in which
the force is applied. Equivalently, since kinetic energy is one half the mass
times the square of the velocity, one joule is the kinetic energy of a mass
of two kilograms moving at a velocity of 1 m/s. This is the same as 107
ergs in the CGS system,
or approximately 0.737 562 foot-pound in the traditional English system. In
other energy units, one joule equals about 9.478 170 x 10-4 Btu,
0.238 846 (small) calories, or 2.777 778
x 10-4 watt hour. The joule is named
for the British physicist James Prescott Joule (1818-1889), who demonstrated
the equivalence of mechanical and thermal energy in a famous experiment in
1843. Although Joule pronounced his name "jowl", the unit is usually pronounced
"jool" or "jew'l".
- joules per kilogram (J/kg)
- the SI unit of specific energy (energy per unit of mass), useful for stating the energy content of foods and fuels. Meteorologists use this unit for CAPE (convective available potential energy), a measure of the instability of the atmosphere critical for predicting the occurrence and strength of thunderstorms.
- a traditional unit of land area in France, equal to the area that could
be plowed in a day (jour is the French word for day). The unit varied
from one region to another, generally in the range 0.3-0.45 hectare
(0.75-1.1 acres). The juchart (see below) was
a very similar unit used in Switzerland and southern Germany.
- abbreviation for Jackson turbidity unit, a unit formerly used in measuring
water quality. The turbidity of water was measured by lighting a candle
under a tall glass tube and filling the tube with the water sample until
an observer looking down the tube could no longer see the candle flame through
the water. The height of the water column determines the turbidity in Jackson
turbidity units, using a table constructed for this purpose. Turbidity is
usually measured now in a different unit, the NTU,
using an instrument called a nephelometer.
- juchart or juchert
- a traditional unit of land area in southern Germany and German-speaking
Switzerland. Like the Austrian joch and the French journal (see above), the
juchart represents an area that could be plowed in a day by a yoke of oxen.
The juchart varied considerably from place to place, but often it was about
4000 square meters, very close to the size of the English acre.
In Bavaria, the juchart was standardized early in the nineteenth century at
3407.27 square meters (0.8420 acre). In Switzerland, after the introduction
of the metric system in the mid nineteenth century, the juchart was generally
understood to equal 3600 square meters (0.8896 acre). The joch, the journal,
and the juchart are ultimately derived from a Roman unit, the jugerum,
which was smaller, about 2500 square meters. The unit is also called the tagwerk
- an informal name for the Scots pint, a unit of volume equal to about 1.80
U.S. liquid quarts or 1.70 liters. Specifically,
the jug of Stirling is the actual vessel (on display at the Stirling
Museum) which was the legal standard for Scottish volume measurements prior
to the introduction of the British Imperial units.
- Julian day (JD)
- a continuous count of days beginning with January 1, 4713 BC (-4712 CE),
which is start of what is called the Julian period. The French
scholar Joseph Justus Scaliger (1540-1609) introduced the Julian period in
1582 (the same year the Gregorian calendar was
proclaimed), defining it to be 7980 years, the product of the 28-year cycle
of the Julian calendar (after which the days of the week recur on the same
dates), the 19-year Metonic cycle (after
which the phases of the Moon recur on the same dates), and the 15-year indiction
cycle (a unit of civil time in ancient Rome). It happens that 4713 BC is the
last year in which all three cycles started simultaneously. In 1849 the British
astronomer John Herschel introduced the Julian day as a means of providing
an exact date for astronomical events independent of all calendars. The Julian
day begins at noon Universal Time, and exact times
of observations are expressed using decimal fractions of the Julian day. The
first moment of the year 2004 CE, Universal Time, was JD 2 453 005.5. See
also modified Julian day.
- Julian epoch (J)
- a measure of time used in astronomy. The word epoch comes from Greek,
and means a fixed or standard instant of time. Other times are stated with
reference to this fixed time using years and fractions of years. In 1984,
astronomers agreed to fix the standard epoch at 12 hours Universal
Time of 2000 January 1 (JD 2 451 545.0). This instant is designated
J2000.0. Other times are specified with reference to this time using a year
of length 365.25 days, the average length of the year in the Julian calendar.
This means that J2001.0, for example, is 18 hours Universal Time of 2001
January 1 (exactly 365.25 days after J2000.0). See Julian year, below.
- Julian period
- a unit of time equal to exactly 7980 years in the Julian calendar, that
is, exactly 2 914 695 days. See Julian day, above.
- Julian year (a)
- the average length of the year in the Julian calendar, equal to exactly
365.25 days. See year . Astronomers
use Julian years in computing the motions of planets and other bodies of
the Solar System. This simplifies their calculations, but the results must
then be translated to give accurate times in other calendars.
- a unit of mass, now being used in astronomy to express the masses of new
planets being discovered in orbit around various stars. It's equal to the
mass of the planet Jupiter, estimated to be about 1.899 x 1024
metric tons, or, if you please, 1.899 yottatonnes (Yt). By coincidence, this
is approximately 0.001 Sun (0.000 955 Sun, to be more exact); it is also
about 317.7 times the mass of the Earth.
- a traditional unit of land area in Croatia, the jutro equals 5754.64 square
meters (1.422 acres), identical to the Austrian
joch (see above) and Czech jitro. This is the area of a square 40 hvati
on a side. The word means "morning," that is, it represents the area that
could be plowed in one morning. The plural is jutra.
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September 18, 2001