Half-Million H.P. Hydroelectric Station at Queenston, Canada Earliest and Latest Forms of Dynamos built in the United States. A Small Electric-Light and Power Plant for Rural Homes Shadow Photograph of a Bullet in Full Flight The Great Telescope of the Yerkes Observatory Section of a Moving-Picture Film. PAGE 362 363 372 373 382 383 390 390 390 390 391 402 403 418 419 Alexander Graham Bell. (Photograph transmitted by wire) 428 Transmission of Pictures by Wire 429 Three-Color Printing. (In colors). 440 Continuous Bright-Line and Absorption Spectra. (In colors) A Bank of Fifteen Water-Cooled Power Amplifier Tubes 460 The Wireless Telephone utilized in Aviation 461 Vacuum Tube used in Early Experiments in Wireless Telephony 466 Comparison of Dry-Cell Tube with Water-Cooled Power Tube ELEMENTS OF PHYSICS CHAPTER I MEASUREMENT FUNDAMENTAL UNITS 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 sink in water, whereas wood floats; that the teakettle stops boiling when removed from the fire; that we wait, after a lightning flash, to hear the thunder; 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. Our accurate knowledge about natural phenomena has been acquired chiefly 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. The yard is supposed to have represented |