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47. The Original Wright Glider and the First Power-Driven Airplane 317 48. Sound Waves of Spoken Words
50. A. A. Michelson
52. Henry A. Rowland
53. Sir William Crookes .
54. X-Ray Picture of the Human Thorax
55. Christian Huygens
56. The Great Telescope of the Yerkes Observatory
57. Section of a "Movie" Film . .
58. Arthur L. Foley's Sound-Wave Photographs
49. Sound Ranging Record of the End of the War
51. Lord Rayleigh (John William Strutt)
61. Cinematograph Film of a Bullet fired through a Soap Bubble
65. Amplifier, and Diagram of Receiving and Amplifying Set.
1. Introductory. A certain amount of knowledge about familiar things comes to us all very early in life. We learn almost unconsciously, for example, that stones fall and balloons rise, that the teakettle stops boiling when removed from the fire, that telephone messages travel by electric currents, etc. The aim of the study of physics is to set us to thinking about how and why such things happen, and, to a less degree, to acquaint us with other happenings which we may not have noticed or heard of previously.
Most of our accurate knowledge about natural phenomena has been acquired through careful measurements. We can measure three fundamentally different kinds of quantities, length, mass, and time, and we shall find that all other measurements may be reduced to these three. Our first problem in physics is, then, to learn something about the units in terms of which all our physical knowledge is expressed.
2. The historic standard of length. Nearly all civilized nations have at some time employed a unit of length the name of which bore the same significance as does foot in English. There can scarcely be any doubt, therefore, that in each country this unit has been derived from the length of
the human foot. It is probable that in England, after the yard (a unit which is supposed to have represented the length of the arm of King Henry I) became established as a standard, the foot was arbitrarily chosen as one third of this standard yard. In view of such an origin it will be clear why no agreement existed among the units in use in different countries.
3. Relations between different units of length. It has also been true, in general, that in a given country the different units of length in common use (such, for example, as the inch, the hand, the foot, the fathom, the rod, the mile, etc.) have been derived either from the lengths of different members of the human body or from equally unrelated magnitudes, and in consequence have been connected with one another by different, and often by very awkward, multipliers. Thus, there are 12 inches in a foot, 3 feet in a yard, 51 yards in a rod, 1760 yards in a mile, etc.
4. Relations between units of length, area, volume, and mass. A similar and even worse complexity exists in the relations of the units of length to those of area, capacity, and mass. Thus, there are 2721 square feet in a square rod; 573 cubic inches in a quart, and 311 gallons in a barrel. Again, the pound, instead of being the mass of a cubic inch or a cubic foot of water, or of some other common substance, is the mass of a cylinder of platinum, of inconvenient dimensions, which is preserved in London.
5. Origin of the metric system. At the time of the French Revolution the extreme inconvenience of existing weights and measures, together with the confusion arising from the use of different standards in different localities, led the National Assembly of France to appoint a commission to devise a more logical system. The result of the labors of this commission was the present metric system, which was introduced in France in 1793 and has since been adopted by the governments of most civilized nations except those of Great Britain and the
United States; and even in these countries its use in scientific work is practically universal. The World War has done much to speed its adoption in these countries.
6. The standard meter. The standard length in the metric system is called the meter. It is the distance, at the freezing temperature, between two transverse parallel lines ruled on a bar of platinum-iridium (Fig. 1), which is kept at the International Bureau of Weights and Measures at Sèvres, near Paris. This distance is 39.37 inches.
In order that this standard length might be reproduced if lost, the commission attempted to make it one ten-millionth
Exact size of the cross section
FIG. 1. The standard meter
of the distance from the equator to the north pole, measured on the meridian of Paris. But since later measurements have thrown some doubt upon the exactness of the commission's determination of this distance, we now define the meter, not as any particular fraction of the earth's quadrant, but simply as the distance between the scratches on the bar mentioned above. On account of its more convenient size, the centimeter, one one-hundredth of a meter, is universally used, for scientific purposes, as the fundamental unit of length.
7. Metric standard capacity. The standard unit of capacity is called the liter. It is the volume of a cube which is one tenth of a meter (about 4 inches) on a side. The liter is therefore
equal to 1000 cubic centimeters (cc.). It is equivalent to 1.057 quarts. A liter and a quart are therefore roughly equivalent
8. The metric standard of mass. In order to establish a connection between the unit of length and the unit of mass, the commission directed a committee of the French Academy to prepare a cylinder of platinum which should have the same weight as a liter of water at its temperature of greatest density, namely, 4° Centigrade (39° Fahrenheit). An exact equivalent of this cylinder, made of platinum-iridium and kept at Sèvres with the standard meter, now represents the standard of mass in the metric system. It is called the standard kilogram and is equivalent to about 2.2 pounds. One one-thousandth of this mass was adopted as the fundamental unit of mass and was named the gram. For practical purposes, therefore, the gram may be taken as equal to the mass of one cubic centimeter of water. 9. The other metric units. The three standard units of the metric system—the meter, the liter, and the gram — have decimal multiples and submultiples, so that every unit of length, volume, or mass is connected with the unit of next higher denomination by an invariable multiplier, namely, ten. The names of the multiples are obtained by adding the Greek prefixes, deka (ten), hecto (hundred), kilo (thousand); while the submultiples are formed by adding the Latin prefixes, deci (tenth), centi (hundredth), and milli (thousandth). Thus:
The most common of these units, with the abbreviations
which will henceforth be used for them,
are the following: