ELECTRICAL UNITS. Two systems of electrical units are in use by electricians, known respectively as the Practical system and the C. G. S. (centimeter-gramme second). system. The former is used in electrical engineering, the latter in the notation of electrical science. The Practical system is sometimes called the Q. E. S. (quadrant-eleventh-second) system. It is based on the earth quadrant, or 10° cen timeters as the unit of length; 10-n gramme as the unit of mass; and the second as the unit of time. The foundational mechanical units from which the Practical system of electrical units is developed are the dyne and the erg. The dyne is a unit of force, assumed to be that force which gives a weight of one gramme a velocity of one centimeter per second during that second. The erg is a unit of work, being the energy exercised when a weight of one gramme is moved a distance of one centimeter with the force of one dyne.
Arranged alphabetically, the principal units of the Practical system with their definitions and their ratios to the units of the same class in the C. G. S. system are as follows: Ampere (the unit of current)— the rate, or volume-per-second, of current flowing through a conductor in which the resistance is one ohm, when the pressure is one volt. It is one-tenth of the C. G. S. unit of current, designated as 10-1— that is, 10 to the —1 power.
Coulomb (the unit of quantity) — the quantity of electricity passing through a con ductor in one second when the rate is one ampere. This unit is not often employed.
Quantity is generally designated in ampere hours, an ampere hour being 3,600 coulombs. The coulomb is C. G. S. units of quantity.
Farad (the unit of capacity) —the capacity of a condenser which would require a charge of one coulomb to establish a difference of potential amounting to one volt between the two conductors forming the condenser. The farad
being expreised by figures inconveniently large, the micro-farad, the one-millionth part of a farad, is commonly used. A farad is equal to 10-' C. G. S. units of capacity.
Henry (the unit of inductance) — the in ductance produced in a circuit when the cur rent is changing at the rate of one ampere per second and is producing in the circuit a dif ference of pressure amounting to one volt. A henry is equal to 10° C. G. S. units of inductance.
Joule (the unit of work) — the work done by one ampere of current flowing for one second through a conductor which has a re sistance of one ohm. The joule is seldom em ployed, the watt hour being the more common unit, equivalent to 3,600joules. (See WATT) . The joule is equal to 10' C. G. S. units of work — that is, 10' ergs.
Ohm (the unit of resistance) — the resist ance of a column of mercury weighing 14.4521 grammes, of such constant cross-section as to be 106.3 centimeters in length. In designating very high resistances the unit used is the equivalent to 1,000,000 ohms. The ohm is C. G. S. units or resistance.
Volt (the unit of pressure) — the electro motive force (E. M. F.) required to force a steady current of one ampere against a re sistance of one ohm. The volt is equal to 10' C. G. S. units of pressure.
Watt (the unit of power) —a current of one ampere flowing under a pressure of one volt — equivalent to one joule per second. The usual commercial unit is the kilowatt, or 1,000 watts. The commercial consumption of electric current is commonly designated as kilowatt hours. The physical unit of power called one horse power is equivalent to 746 watts. The i watt equal to 10' C. G. S. units of power— that is, 10' ergs-per-second.