(1) Wireless Telephony 460 (2) Alternating Currents 466 INDEX 487 PHYSICS CHAPTER I INTRODUCTION AND MEASUREMENT (1) INTRODUCTION =1. Physics, an Explanation of Common Things.=--Many students take up the study of physics expecting to see wonderful experiments with the "X" rays, wireless telegraphy, dynamos, and other interesting devices. Others are dreading to begin a study that to them seems strange and difficult, because they fear it deals with ideas and principles that are beyond their experience and hard to comprehend. Each of these classes is surprised to learn that _physics is mainly an explanation of common things_. It is a study that systematizes our knowledge of the forces and changes about us; such as the pull of the earth, the formation of dew, rain and frost, water pressure and pumps, echoes and music, thermometers and engines, and many other things about us with which people are more or less familiar. Physics is like other school subjects, such as mathematics and language, in having its own peculiar vocabulary and methods of study; these will be acquired as progress is made in the course. The most useful habit that the student of physics can form is that of connecting or relating each _new idea_ or _fact_ that is presented to him to _some observation_ or _experience_ that will illustrate the new idea. This relating or connecting of the new ideas to one's own personal experience is not only one of the best known means of cultivating the memory and power of association, but it is of especial help in a subject such as physics, which deals with the systematic study and explanation of the facts of our every-day experience. =2. Knowledge--Common and Scientific.=--This leads to the distinction between _common knowledge and scientific knowledge_. We all possess common knowledge of the things about us, gained from the impressions received by our senses, from reading, and from the remarks of others. _Scientific_ knowledge is attained when the bits of common knowledge are connected and explained by other information gained through study or experience. That is, common knowledge becomes scientific, when it is _organized_. This leads to the definition: _Science is organized knowledge_. Common knowledge of the forces and objects about us becomes scientific only as we are able to make accurate measurements of these. That is, science is concerned not only in _how_ things work, but even more in _how much_ is involved or results from a given activity. For example, a scientific farmer must be able to compute his costs and results in order to determine accurately his net profits. The business man who is conducting his business with efficiency knows accurately his costs of production and distribution.