## How Many? A Dictionary of Units of Measurement © Russ Rowlett and the University of North Carolina at Chapel Hill Table of Contents About the Dictionary Using the Dictionary J

jag
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.
jar
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).
jerk
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 /sec.
jeroboam
a large wine bottle holding about 3 liters, 4 times the volume of a regular bottle. A jeroboam is sometimes called a double magnum.
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 [1]
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 [2]
a unit of time used in chemistry and physics, equal to a "light centimeter," 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.
jigger
a unit of volume for liquor, usually considered equal to 1.5 (U.S.) fluid ounces or 44.360 milliliters.
jin
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.
jitro
a traditional unit of land area in the Czech Republic, identical to the Austrian joch (see below).
jo
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).
joch
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.
Jones
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".
journal
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.
JTU
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 ("day's work").
jug
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 [2]. 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.
Jupiter
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.
jutro
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.