# Physics : Measurement Units

**Measurement Units**

**Acre:** A measure of land, 4,840 square yards (4,046 square metres).

**Ampere:** Unit for measuring the strength of an electric current. It is the amount of current sent by one volt through a resistance of one ohm.

**Angstrom:** It is the unit for measuring the wavelength of light. It is one hundred-millionth of a centimetre.

**Astronomical unit:** A unit of length equal to the mean radius of earth orbit. It is 149,597,870 km (92,955,800 miles).

**Bar:** Unit of atmospheric pressure. One bar is equal to a pressure of 106 dynes per sq cm.

**Barrel:** For measuring liquids. One barrel is equal to 31.1/2 gallons in US and 36 imperial gallons in Britain.

**Bushel:** Unit of dry measure for grain, fruit, etc. It is equal to 4 pecks or 8 gallons.

**Calorie:** Unit for measuring the amount of heat required to raise the temperature of one gram of water through 1°C. It is used as the unit for measuring the energy produced by food when oxidised in the body.

**Carat:** Unit of weight for precious stones and pearls. It is equal to about 3.17 grains troy or .2 of gram. It is also a measure of the purity of gold alloy indicating how many parts out of 24 are pure.

**Coulomb:** Unit for measuring the quantity of an electric current. It is the amount of electricity provided by a current of one ampere flowing for one second.

**Decibel:** Unit for measuring the volume of sound. It is equal to the logarithm of the ratio of the intensity of sound to the intensity of an arbitrarily chosen standard sound.

**Dyne:** Amount of force that causes a mass of one gram to alter its speed by one centimetre per second for each second during which the force acts. This unit of force is in CGS (metric) system.

**Erg:** Unit of work or energy in CGS (metric) system. It is the amount of work done by one dyne acting through a distance of one centimetre.

**Farad:** Electromagnetic unit of capacitance. It is equal to the amount that permits the storing of one coulomb of charge for each volt of applied potential difference.

**Fathom:** Unit for measuring the depth of water or the length of a rope or cable. One fathom is equal to 6 feet.

**Gallon:** It is a measure of liquid.

**Gross:** 12 dozens or 144.

**Hertz:** Modern unit for measurement of electromagnetic wave frequencies.

**Horse power:** Unit for measuring the power of motors or engines. It is equal to a rate of 33,000 foot-pounds per minute, i.e., the force required to raise 33,000 pounds at the rate of one foot per minute. HP = 746 watts

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