Elementary Text-book of Physics/Introduction


1. Divisions of Natural Science.—Everything which can affect our senses we call matter. Any limited portion of matter, however great or small, is called a body. All bodies, together with their unceasing changes, constitute Nature.

Natural Science makes us acquainted with the properties of bodies, and with the changes, or phenomena, which result from their mutual actions. It is therefore conveniently divided into two principal sections,—Natural History and Natural Philosophy.

The former describes natural objects, classifies them according to their resemblances, and, by the aid of Natural Philosophy, points out the laws of their production and development. The latter is concerned with the laws which are exhibited in the mutual action of bodies on each other.

These mutual actions are of two kinds: those which leave the essential properties of bodies unaltered, and those which effect a complete change of properties, resulting in loss of identity. Changes of the first kind are called physical changes; those of the second kind are called chemical changes. Natural Philosophy has, therefore, two subdivisions,—Physics and Chemistry.

Physics deals with all those phenomena of matter which are not directly related to chemical changes. Astronomy is thus a branch of Physics, yet it is usually excluded from works like the present on account of its special character.

lt is not possible, however, to draw sharp lines of demarcation between the variisus departments of Natural Science, for the successful pursuit of knowledge in any one of them requires some acquaintance with the others.

2. Methods.—The ultimate basis of all our knowledge of Nature is experience,—experience resulting from the action of bodies on our senses, and the consequent affections of our minds.

When a natural phenomenon arrests our attention, we call the result an observation. Simple observations of natural phenomena only in rare instances can lead to such complete knowledge as will suffice for a full understanding of them. An observation is the more complete, the more fully we apprehend the attending circumstances. We are generally not certain that all the circumstances which we note are conditions on which the phenomenon, in a given case, depends. In such cases we modify or suppress one of the circumstances, and observe the effect on the phenomenon. If we find a corresponding modification or failure with respect to the phenomenon, we conclude that the circumstance, so modified, is a condition. We may proceed in the same way with each of the remaining circumstances, leaving all unchanged except the single one purposely modified at each trial, and always observing the effect of the modification. We thus determine the conditions on which the phenomenon depends. In other words, we bring experiment to our aid in distinguishing between the real conditions on which a phenomenon depends, and the merely accidental circumstances which may attend it.

But this is not the only use of experiment. By its aid we may frequently modify some of the conditions, known to be conditions, in such ways that the phenomenon is not arrested, but so altered in the rate with which its details pass before us that they may be easily observed. Experiment also often leads to new phenomena, and to a knowledge of activities before unobserved. Indeed, by far the greater part of our knowledge of natural phenomena has been acquired by means of experiment. To be of value, experiments must be conducted with system, and so as to trace out the whole course of the phenomenon.

Having acquired our facts by observation and experiment we seek to find out how they are related; that is, to discover the laws which connect them. The process of reasoning by which we discover such laws is called induction. As we can seldom be sure that we have apprehended all the related facts, it is clear that our inductions must generally be incomplete. Hence it follows that oonclusions reached in this way are at best only probable; yet their probability becomes very great when we can discover no outstanding fact, and especially so when, regarded provisionally as true, they enable us to predict phenomena before unknown.

In conducting our experiments, and in reasoning upon them, we are often guided by suppositions suggested by previous experience. If the course of our experiment be in accordance with our supposition, there is, so far, a presumption in its favor. So, too, in reference to our reasonings: if all our facts are seen to be consistent with some supposition not unlikely in itself, we say it thereby becomes probable. The term hypothesis is usually employed instead of supposition.

Concerning the ultimate modes of existence or action, we know nothing whatever; hence, a law of nature cannot be demonstrated in the sense that a mathematical truth is demonstrated. Yet so great is the constancy of uniform sequence with which phenomena occur in accordance with the laws which we discover, that we have no doubt respecting their validity.

When we would refer a series of ascertained laws to some common agency, we employ the term theory. Thus we find in the "wave theory " of light, based on the hypothesis of a universal ether of extreme elasticity, satisfactory explanations of the laws of reflection, refraction, difEraction, polarization, etc.

3. Measurements.—All the phenomena of Nature occur in matter, and are presented to us in time and space. Time and space are fundamental conceptions: they do not admit of definition. Matter is equally indefinable: its distinctive characteristic is its persistence in whatever state of rest or motion it may happen to have, and the resistance which it offers to any attempt to change that state. This property is called inertia. It must be carefully distinguished from inactivity.

Another essential property of matter is impenetrability, or the property of occupying space to the exclusion of other matter.

We are almost constantly obliged, in physical science, to measure the quantities with which we deal. We measure a quantity when we compare it with some standard of the same kind. A simple number expresses the result of the comparison.

If we adopt arbitrary units of length, time, and mass (or quantity of matter), we can express the measure of all other quantities in terms of these so-called fundamental units. A unit of any other quantity, thus expressed, is called a derived unit.

It is convenient, in defining the measure of derived units, to speak of the ratio between, or the product of, two dissimilar quantities, such as space and time. This must always be understood to mean the ratio between, or the product of, the numbers expressing those quantities in the fundamental units. The result of taking such a ratio or product of two dissimilar quantities is a number expressing a third quantity in terms of a derived unit.

4. Unit of Length.—The unit of length usually adopted in scientific work is the centimetre. It is the one hundredth part of the length of a certain piece of platinum, declared to be a standard by legislative act, and preserved in the archives of France. This standard, called the metre, was designed to be equal in length to one ten-millionth of the earth's quadrant.

The operation of comparing a length with the standard is often difficult of direct accomplishment. This may arise from the minuteness of the object or distance to be measured, from the distant point at which the measurement is to end being inaccessible, or from the difficulty of accurately dividing our standard into very small fractional parts. In all such cases we have recourse to indirect methods, by which the difficulties are more or less completely obviated.

The vernier enables us to estimate small fractions of the unit of length with great convenience and accuracy. It consists of an accessory piece, fitted to slide on the principal scale of the instrument to which it is applied.
A portion of the accessory piece, equal to minus one or plus one divisions of the principal scale, is divided into divisions. In the former case, the divisions are numbered in the same sense as those of the principal scale; in the latter, they are numbered in the opposite sense. In either case we can measure a quantity accurately to the one nth part of one of the primary divisions of the principal scale. Fig. 1 will make the construction and use of the vernier plain.

In Fig. 1, let 0, 1, 2, 3 ... 10 be the divisions on the vernier; let 0, 1, 2, 3 . . . 10 be any set of consecutive divisions on the principal scale.

If we suppose the of the vernier to be in coincidence with the limiting point of the magnitude to be measured, it is clear that, from the position shown in the figure, we have 29.7, expressing that magnitude to the nearest tenth; and since the sixth division of the vernier coincides with a whole division of the principal scale, we have of , or of a principal division to be added: hence the whole value is 29.76.

The micrometer screw is also much employed. It consists of a carefully cut screw, accurately fitting in a nut. The head of the screw carries a graduated circle, which can turn past a fixed line. This is frequently the straight edge of a scale with divisions equal in magnitude to the pitch of the screw. These divisions will then show through how many revolutions the screw is turned in any given trial; while the divisions on the graduated circle will show the fractional part of a revolution, and consequently the fractional part of the pitch that must be added. If the screw be turned through revolutions, as shown by the scale, and through an additional fraction, as shown by the divided circle, it will pass through times the pitch of the screw, and an additional fraction of the pitch determined by the ratio of the number of divisions read from 0 on the divided circle to the whole number into which it is divided.

The cathetometer is used for measuring differences of level. A graduated scale is cut on an upright bar, which can turn about a vertical axis. Over this bar slide two accurately fitting pieces, one of which can be clamped to the bar at any point, and serve as the fixed bearing of a micrometer screw. The screw runs in a nut in the second piece, which has a vernier attached, and carries a horizontal telescope furnished with cross-hairs. The telescope having been made accurately horizontal by means of a delicate level, the cross-hairs are made to cover one of the two points, the difference of level between which is sought, and the reading upon the scale is taken; the fixed piece is then undamped, and the telescope raised or lowered until the second point is covered by the cross-hairs, and the scale reading is again taken. The difference of scale reading is the difference of level sought.

The dividing engine may be used for dividing scales or for comparing lengths. In its usual form it consists essentially of a long micrometer screw, carrying a table, which slides, with a motion accurately parallel with itself, along fixed guides, resting on a firm support. To this table is fixed an apparatus for making successive cuts upon the object to be graduated.

The object to be graduated is fastened to the fixed support. The table is carried along through any required distance determined by the motion of the screw, and the cuts can be thus made at the proper intervals.

The same instrument, furnished with microscopes and accessories, may be employed for comparing lengths with a standard. It may then be called a comparator.

The spherometer is a special form of the micrometer screw. As
its name implies, it is primarily used for measuring the curvature of spherical surfaces.

It consists of a screw with a large head, divided into a great number of parts, turning in a nut supported on three legs terminating in points, which form the vertices of an equilateral triangle. The axis of revolution of the screw is perpendicular to the plane of the triangle, and passes through its centre. The screw ends in a point which may be brought into the same piano with the points of the legs. This is done by placing the legs on a truly plane surface, and turning the screw till its point is just in contact with the surface. The sense of touch will enable one to decide with great nicety when the screw is turned far enough. If, now, we note the reading of the divided scale and also that of the divided head, and then raise the screw, by turning it backward, so that the given curved surface may exactly coincide with the four points, we can compute the radius of curvature from the difference of the two readings and the known length of the side of the triangle formed by the points of the tripod.

5. Unit of Time.— The unit of time is the mean time second, which is the of a mean solar day. We employ the clock, regulated by the pendulum or the chronometer balance, to indicate seconds. The clock, while sufficiently accurate for ordinary use, must for exact investigations be frequently corrected by astronomical observations.

Smaller intervals of time than the second are measured by causing some vibrating body, as a tuning-fork, to trace its path along some suitable surface, on which also are recorded the beginning and end of the interval of time to be measured. The number of vibrations traced while the event is occurring determines its duration in known parts of a second.

In estimating the duration of certain phenomena giving rise to light, the revolving mirror may be employed. By its use, with proper accessories, intervals as small as forty billionths of a second have been estimated.

6. Unit of Mass.— The unit of mass usually adopted in scientific work is the gram. It is equal to the one-thousandth part of a certain piece of platinum, called the kilogram, preserved as a standard in the archives of France. This standard was intended to be equal in mass to one cubic decimetre of water at its greatest density.

Masses are compared by means of the balance, the construction of which will be discussed hereafter.

7. Measurement of Angles.— Angles are usually measured by reference to a divided circle graduated on the system of division upon which the ordinary trigonometrical tables are based. A pointer or an arm turns about the centre of the circle, and the angle between two of its positions is measured in degrees on the arc of the circle. For greater accuracy, the readings may be made by the help of a vernier. To facilitate the measurement of an angle subtended at the centre of the circle by two distant points, a telescope with cross-hairs is mounted on the movable arm.

In theoretical discussions the unit of angle often adopted is the radian, that is, the angle subtended by the arc of a cirule equal to its radius. In terms of this unit, a semi-circumference equals π = 3.141592. The radian, measured in degrees, is 57° 17' 44.8."

8. Dimensions of Units.— Any derived unit may be represented by the product of certain powers of the symbols representing the fundamental units of length, mass, and time.

Any equation showing what powers of the fundamental units enter into the expression for the derived unit is called its dimensional equation. In a dimensional equation time is represented by , length by , and mass by . To indicate the dimensions of any quantity, the symbol representing that quantity is enclosed in brackets.

For example, the unit of area varies as the square of the unit of length; hence its dimensional equation is . In like manner, the dimensional equation for volume is .

9. Systems of Units.— The system of units adopted in this book, and generally employed in scientific work, based upon the centimetre, gram, and second, as fundamental units, is called the centimetre-gram-second system or the C. G. S. system. A system based upon the foot, grain, and second was formerly much used in England. One based upon the millimetre, milligram, and second is still sometimes used in Germany.