UNITS

Puystcan, are those quantities in terms of which measurements are expre ed. Dynamical Units——In the Amer. absolute system the fundamental units are the foot (length), pound (ma ), and second (time), the unit of force being the poundal or force which, acting on one pound for one second, generates a et ce ons foot per second. This i force is — or Sq of the force with which g fi the earth attracts one pound weight at sea level at Green wich. The unit of work is the foot-poundal. Owing to complex tables of weights and measures, calculations in the above systems are somewhat laborious. In the gravitation or engineer’s system, with the same fundamental units of length and time, the unit force is that due to the weight of one pound. The unit of ma is therefore that of 34.2 lb. The unit of work is the foot pound. The metric system and the c.a.s. (centimetre-gram-second) system are now becoming universal. The units are connected simply, and as_ decimal notation is used, calculation becomes easy. It has the disadvantage that its units of force and work (dyne and erg) are extremely small, and hence many quantities used by practical men have to be expre ed in very large numbers. Gravitational units, however, are also used, the weight of one gram or a kilogram (1,000 grams) being used as a unit of force. Units of surface and volume are obtained by squaring and cubing the units of length. Derived units are those of velocity, acceleration, force, etc. AS an example of a derived unit, the unit of force is that force which, acting on unit ma for unit time, gives to it unit velocity—.e., a velocity of unit distance in unit time. Dimensions of Uniis.—A length (1) 4s of 1 dimension, an area (12) of 2 dimensions, and a volume (13) of 3 dimensions. <A velocity (length +time) L — or tt! is of 1 dimension in length and T {(-1) in time. The dimensions of other units are derived similarly: for instance, a force is mtT—~, and work is mi2 TT. From the c.a.s. system the two systems of electrical units, electrostatic and electro-magnetic, are derived. The ‘former is based on the force exerted between two quantities of electricity, the latter on the force between two magnetic poles. Hlectrostatic Units —Unit quantity of electricity is chosen as that which repels a similar quantity at unit distance with unit force. Unit current conveys unit quantity in unit time, while unit H.M.F. exists between two points when unit current flowing between them does unit work. Hence the units of resistance and capacity are derived. Magnetic Units —Unit magnetic pole repels unit like pole in air with unit force. The strength of a magnetic field is measured by the force acting on unit magnetic pole at that point, unit field acting on unit pole with unit force. Electro-magnetic Units.—Unit current is that current which, in a circular arc of unit length and unit radius, acts on unit pole at the center with unit force. Unit quantity is conveyed by unit current in unit time. Unit &.m.r. is generated in a conductor of unit length moving with unit velocity in unit field. From these the units of resistance and capacity are derived.