SYNOPSIS OF METRIC SYSTEM AND EQUIVALENTS. [From a domestic catalogue.] THE METRIC SYSTEM. The metric system is based on the meter which was designed to be one ten-millionth (1-10,000,000) part of the earth's meridian, passing through Dunkirk and Formentera. Later investigations, however, have shown that the meter exceeds one ten-millionth part by almost one part in 6,400. The value of the meter, as authorized by the United States Government, is 39.37 inches. The metric system was legalized by the United States Government in 1866. The three principal units are the meter, the unit of length; the liter, the unit of capacity; and the gram, the unit of weight. Multiples of these are obtained by prefixing the Greek words: deka (10), hekto (100), and kilo (1,000). Divisions are obtained by prefixing the Latin words: deci (1-10), centi (1-100), and milli (1-1,000). Abbreviations of the multiples begin with a capital letter, and of the divisions with a small letter, as in the following tables: NOTE. The liter is equal to the volume occupied by 1 cubic decimeter. .cl .dl ..1 .DI HI Kl NOTE.-The gram is the weight of 1 cubic centimeter of pure distilled water at a temperature of 39.2° F.; the kilogram is the weight of 1 liter of water; the ton is the weight of 1 cubic meter of water. METRIC AND ENGLISH OR AMERICAN (U. S.) EQUIVALENT MEASURES. 1 kilogram per meter=.6720 pound per foot. 1 gram per square millimeter-1.422 pounds per square inch. 1 kilogram per square meter=0.2084 pound per square foot. 1 kilogram per cubic meter=.0624 pound per cubic foot. 1 degree centigrade=1.8 degrees Fahrenheit. 1 pound per foot=1.488 kilograms per meter. 1 pound per square foot=4.882 kilograms per square meter. 1 pound per cubic foot=16.02 kilograms per cubic meter. 1 degree Fahrenheit=.5556 degree centigrade. 1 calorie (French thermal unit)=3.968 B. t. u. (British thermal unit). 33,000 foot pounds per minute. 1 horsepower = 746 watts. .00134 horsepower. 1 watt (unit of electrical power)=44.24 foot pounds per minute. 1,000 watts. 1 kilowatt-1.34 horsepower. (44,240 foot pounds per minute. Unit. 1 h. p. 1h. p. hour= 1 kilowatt= THE METRIC SYSTEM-Continued. WEIGHTS-Continued. to 212° F. 1,000 watts. 1.34 h. p. 2,654,200 ft.-lbs. per hour. 44,240 ft.-lbs. per minute. 737.3 ft.-lbs. per second. 3,412 heat units per hour. 56.9 heat units per minute. .948 heat unit per second. .2275 lb. carbon oxidized per hour. 3.53 lbs. water evaporated per hour from and at 212° F. 8.19 heat units per sq. ft. per minute. 6,371 ft.-lbs. per sq. ft. per minute. .193 h. p. per sq. ft. 1 watt per sq.) in.= Unit. 1 heat unit= 1 heat unit per sq. ft. per min.= 1 watt Equivalent value in other units. 107.6 kilogram meters. .00006881b. carbon oxidized 3,412 heat units per hour. 44.24 ft.-lbs. per minute. 1,000 watt hours. 1.34 h. p. hours. 2,654,200 ft.-lbs. 3,600,000 joules. 3,412 heat units. 1 k. w. hour 367,000 kilogram meters. 1 joule= 1 ft.-lb. 1 lb. carbon oxidized) with perfect efficiency= .235 lb. carbon oxidized with perfect efficiency. 3.53 lbs. water evaporated from and at 212° F. 22.75 lbs. of water raised from 62° to 212° F. METRIC UNITS USED IN ELECTRICITY DEFINED IN A UNITED STATES CATALOGUE. USEFUL Data. DEFINITIONS OF ELECTRICAL UNITS. All electrical units are derived from the following mechanical units: The centimeter is the unit of length, and equals .3937 inch, or .000000001 of a quadrant of the earth. The gram is the unit of mass, and is equal to 15.432 grains, the mass of a cubic centimeter of water at 4° C. The second is the unit of time and is the time of one swing of a pendulum, swinging 86464.09 times per day, or the 1-86400th part of a mean solar day. The volt is the unit of electro-motive force (E). Electro-motive force, which is the force that moves electricity, is usually written E. M. F. (in formulæ E) and various writers use it to express potential difference of potential, electric pressure, and electric force. One volt will force an ampere of current through one ohm of resistance. Its value is purely arbitrary, but fixed. The ohm is the unit of resistance (R), and it is equal to the resistance of a column of pure mercury 1 square millimeter in section and 106 centimeters long at the temperature of melting ice. One ohm is that resistance through which one ampere of current will flow at a pressure of one volt of E. M. F. The megohm=1,000,000 ohms. The ampere is the unit of current strength (C). Its value may be defined as that quantity of electricity which flows through one ohm of resistance when impelled by one volt of E. M. F. One ampere of current flowing through a bath will deposit 0.017253 grain of silver or 0.005085 grain of copper per second. The coulomb is the unit of quantity (Q), and is the quantity of electricity passing per second when the current is one ampere. The farad is the unit of capacity (K), and is that capacity that will contain one coulomb at a potential of one volt. A condenser of one farad capacity, if charged to two volts, will contain two coulombs; if to 100 volts, 100 coulombs, etc. The microfarad (mfd)=one-millionth of a farad. The joule is the unit of work (W). It is the work done or heat generated by a watt in a second. It is equal to .7373 foot-pound. The watt is the unit of electrical power (P), and is the energy contained in a current of one ampere with an electro-motive force of one volt. 746 watts one horsepower. A current of 10 amperes and 74.6 volts will do the work of one horsepower. A horsepower in a steam engine or other prime mover is 550 lbs. raised one foot per second, or 33,000 lbs. one foot per minute. A kilowatt (kw) equals 1,000 watts. The E. M. F. is distributed according to the resistance of the various parts of the circuit, except where there is counter E. M. F. Counter E. M. F. is like back pressure in hydraulics. Thus to find the available E. M. F., or the resulting current against a resistance where there is a counter E. M. F., the counter E. M. F. must be deducted. For example: Suppose a storage battery with a resistance of .02 ohm and a C. E. M. F. of 15 volts, and you wish to charge it with a dynamo which gives an E. M. F. of 20 volts at the battery binding posts. There are 20-15-5 volts working through a resistance of .02 of an ohm with consequently a current of 250 amperes. The fall of potential is, however, virtually 20 volts, and not 5 volts, and the power is 20X250=5000 watts, and not 5×250=1250 watts, as might perhaps be supposed. It is obvious that the C. E. M. F. has acted as a true resistance. In the above case 5X250-1250 watts were wasted in overcoming the resistance of the storage battery and the remaining 3750 watts were stored up in the chemical changes which they brought about in the active material of the storage battery. Mils. thousandths of an inch. d2=circular mils. The circular mil. is now generally used as the unit of area when considering the cross-section of electric conductors, the resistance being inversely and weight of copper directly proportional to the circular mils. |