_The above arrangement is a dynamo in miniature_. By rotation of a coil (armature) within a magnetic field, that is, between the poles of a magnet, current is obtained.

In the _motor_, current produces motion. In the _dynamo_, motion produces current.

321. The Dynamo. As has been said, the arrangement of the preceding Section is a dynamo in miniature. Every dynamo, no matter how complex its structure and appearance, consists of a coil of wire which can rotate continuously between the poles of a strong magnet. The mechanical devices to insure easy rotation are similar in all respects to those previously described for the motor.

[Ill.u.s.tration: FIG. 239.--A modern electrical machine.]

The current obtained from such a dynamo alternates in direction, flowing first in one direction and then in the opposite direction.

Such alternating currents are unsatisfactory for many purposes, and to be of service are in many cases transformed into direct currents; that is, current which flows steadily in one direction. This is accomplished by the use of a commutator. In the construction of the motor, continuous _motion_ in one direction is obtained by the use of a commutator (Section 310); in the construction of a dynamo, continuous _current_ in one direction is obtained by the use of a similar device.

322. Powerful Dynamos. The power and efficiency of a dynamo are increased by employing the devices previously mentioned in connection with the motor. Electromagnets are used in place of simple magnets, and the armature, instead of being a simple coil, may be made up of many coils wound on soft iron. The speed with which the armature is rotated influences the strength of the induced current, and hence the armature is run at high speed.

[Ill.u.s.tration: FIG. 240.--Thomas Edison, one of the foremost electrical inventors of the present day.]

A small dynamo, such as is used for lighting fifty incandescent lamps, has a horse power of about 33.5, and large dynamos are frequently as powerful as 7500 horse power.

323. The Telephone. When a magnet is at rest within a closed coil of wire, as in Section 319, current does not flow through the wire. But if a piece of iron is brought near the magnet, current is induced and flows through the wire; if the iron is withdrawn, current is again induced in the wire but flows in the opposite direction. As iron approaches and recedes from the magnet, current is induced in the wire surrounding the magnet. This is in brief the principle of the telephone. When one talks into a receiver, _L_, the voice throws into vibration a sensitive iron plate standing before an electromagnet. The back and forth motion of the iron plate induces current in the electromagnet _c_. The current thus induced makes itself evident at the opposite end of the line _M_, where by its magnetic attraction, it throws a second iron plate into vibrations. The vibrations of the second plate are similar to those produced in the first plate by the voice. The vibrations of the far plate thus reproduce the sounds uttered at the opposite end.

[Ill.u.s.tration: FIG. 241.--Diagram of a simple telephone circuit.]

324. Cost of Electric Power. The water power of a stream depends upon the quant.i.ty of water and the force with which it flows. The electric power of a current depends upon the quant.i.ty of electricity and the force under which it flows. The unit of electric power is called the watt; it is the power furnished by a current of one ampere with a voltage of one volt.

One watt represents a very small amount of electric power, and for practical purposes a unit 1000 times as large is used, namely, the kilowatt. By experiment it has been found that one kilowatt is equivalent to about 1-1/3 horse power. Electric current is charged for by the watt hour. A current of one ampere, having a voltage of one volt, will furnish in the course of one hour one watt hour of energy.

Energy for electric lighting is sold at the rate of about ten cents per kilowatt hour. For other purposes it is less expensive. The meters commonly used measure the amperes, volts, and time automatically, and register the electric power supplied in watt hours.

PLANT LIFE AND PLANT USES

By JOHN g.a.y.l.o.r.d COULTER, Ph. D.

An elementary textbook providing a foundation for the study of agriculture, domestic science, or college botany. But it is more than a textbook on botany--it is a book about the fundamentals of plant life and about the relations between plants and man. It presents as fully as is desirable for required courses in high schools those large facts about plants which form the present basis of the science of botany. Yet the treatment has in view preparation for life in general, and not preparation for any particular kind of calling.

The subject is dealt with from the viewpoint of the pupil rather than from that of the teacher or the scientist. The style is simple, clear, and conversational, yet the method is distinctly scientific, and the book has a cultural as well as a practical object.

The text has a unity of organization. So far as practicable the familiar always precedes the unfamiliar in the sequence of topics, and the facts are made to hang together in order that the pupil may see relationships. Such topics as forestry, plant breeding, weeds, plant enemies and diseases, plant culture, decorative plants, and economic bacteria are discussed where most pertinent to the general theme rather than in separate chapters which destroy the continuity. The questions and suggestions which follow the chapters are of two kinds; some are designed merely to serve as an aid in the study of the text, while others suggest outside study and inquiry. The cla.s.sified tables of terms which precede the index are intended to serve the student in review, and to be a general guide to the relative values of the facts presented. More than 200 attractive ill.u.s.trations, many of them original, are included in the book.

AMERICAN BOOK COMPANY

A NEW ASTRONOMY.

By DAVID TODD, M. A., Ph. D., Professor of Astronomy and Navigation and Director of the Observatory, Amherst College.

Astronomy is here presented as preeminently a science of observation.

More of thinking than of memorizing is required in its study, and greater emphasis is laid on the physical than on the mathematical aspects of the science. As in physics and chemistry, the fundamental principles are connected with tangible, familiar objects, and the student is shown how he can readily make apparatus to ill.u.s.trate them.

In order to secure the fullest educational value, astronomy is regarded as an inter-related series of philosophic principles.

MATHEMATICAL GEOGRAPHY.

By WILLIS E. JOHNSON, Ph. D., Vice-President and Professor of Geography and Social Sciences, Northern Normal and Industrial School, Aberdeen, South Dakota.

This work explains with great clearness and thoroughness that portion of the subject which not only is most difficult to understand, but also underlies and gives meaning to all geographical knowledge. A vast number of facts which are much inquired about, but little known, are taken up and explained. Simple formulas are given so that a student unacquainted with geometry or trigonometry may calculate the heights and distances of objects, the lat.i.tude and longitude of a place, the amount any body is lightened by the centrifugal force due to rotation, the deviation of a plumb-line from a true vertical, etc.

AMERICAN BOOK COMPANY

ELEMENTS OF GEOLOGY

By ELIOT BLACKWELDER, a.s.sociate Professor of Geology, University of Wisconsin, and HARLAN H. BARROWS, a.s.sociate Professor of General Geology and Geography, University of Chicago.

An introductory course in geology, complete enough for college cla.s.ses, yet simple enough for high school pupils. The text is explanatory, seldom merely descriptive, and the student gains a knowledge not only of the salient facts in the history of the earth, but also of the methods by which those facts have been determined. The style is simple and direct. Few technical terms are used. The book is exceedingly teachable.

The volume is divided into two parts, physical geology and historical geology. It differs more or less from its predecessors in the emphasis on different topics and in the arrangement of its material. Factors of minor importance in the development of the earth, such as earthquakes, volcanoes, and geysers, are treated much more briefly than is customary. This has given s.p.a.ce for the extended discussion of matters of greater significance. For the first time an adequate discussion of the leading modern conceptions concerning the origin and early development of the earth is presented in an elementary textbook.

The ill.u.s.trations and maps, which are unusually numerous, really ill.u.s.trate the text and are referred to definitely in the discussion.

They are admirably adapted to serve as the basis for cla.s.sroom discussion and quizzes, and as such const.i.tute one of the most important features of the book. The questions at the end of the chapters are distinctive in that the answers are in general not to be found in the text. They may, however, be reasoned out by the student, provided he has read the text with understanding.

AMERICAN BOOK COMPANY

ESSENTIALS OF BIOLOGY

By GEORGE WILLIAM HUNTER, A. M., Head of Department of Biology, De Witt Clinton High School, New York City.

This new first-year course treats the subject of biology as a whole, and meets the requirements of the leading colleges and a.s.sociations of science teachers. Instead of discussing plants, animals, and man as separate forms of living organisms, it treats of fife in a comprehensive manner, and particularly in its relations to the progress of humanity. Each main topic is introduced by a problem, which the pupil is to solve by actual laboratory work. The text that follows explains and ill.u.s.trates the meaning of each problem. The work throughout aims to have a human interest and a practical value, and to provide the simplest and most easily comprehended method of demonstration. At the end of each chapter are lists of references to both elementary and advanced books for collateral reading.