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PREFACE.

This book is intended for the assistance of Students and Teachers in Physical Laboratories. The absence of any book covering the same ground made it necessary for us, in conducting the large elementary classes in Practical Physics at the Cavendish Laboratory, to write out in MS. books the practical details of the different experiments. The increase in the number of well-equipped Physical Laboratories has doubtless placed many teachers in the same position as we ourselves were in before these books were compiled ; we have therefore collected together the manuscript notes in the present volume, and have added such general explanations as seemed necessary.

In offering these descriptions of experiments for publication we are met at the outset by a difficulty which may prove serious. The descriptions, in order to be precise, must refer to particular forms of instruments, and may therefore be to a certain extent inapplicable to other instruments of the same kind but with some difference, perhaps in the arrangement for adjustment, perhaps in the method of graduation. Spherometers, spectrometers, and kathetometers are instruments with which this difficulty is particularly likely to occur. With considerable diffidence we have thought it best to adhere to the precise descriptions referring

to instruments in use in our own Laboratory, trusting that the necessity for adaptation to corresponding instruments used elsewhere will not seriously impair the usefulness of the book. Many of the experiments, however, which we have selected for description require only very simple apparatus, a good deal of which has in our case been constructed in the Laboratory itself. We owe much to Mr. G. Gordon, the Mechanical Assistant at the Cavendish Laboratory, for his ingenuity and skill in this respect

Our general aim in the book has been to place before the reader a description of a course of experiments which shall not only enable him to obtain a practical acquaintance with methods of measurement, but also as far as possible illustrate the more important principles of the various subjects. We have not as a rule attempted verbal explanations of the principles, but have trusted to the ordinary physical text-books to supply the theoretical parts necessary for understanding the subject; but whenever we have not been able to call to mind passages in the text-books sufficiently explicit to serve as introductions to the actual measurements, we have either given references to standard works or have endeavoured to supply the necessary information, so that a student might not be asked to attempt an experiment without at least being in a position to find a satisfactory explanation of its method and principles. In following out this plan we have found it necessary to interpolate a considerable amount of more theoretical information. The theory of the balance has been given in a more complete form than is usual in mechanical text-books; the introductions to the measurement of fluid pressure, thermometry, and calorimetry have been inserted in order to accentuate certain important practical points which, as a rule, are only briefly touched upon ;

while the chapter on hygrometry is intended as a complete elementary account of the subject. We have, moreover, found it necessary to adopt an entirely different style in those chapters which treat of magnetism and electricity. These subjects, regarded from the point of view of the practical measurement of magnetic and electric quantities, present a somewhat different aspect from that generally taken. We have accordingly given an outline of the general theory of these subjects as developed on the lines indicated by the electro-magnetic system of measurement, and the arrangement of the experiments is intended, as far as possible, to illustrate the successive steps in the development. The limits of the space at our disposal have compelled us to be as concise as possible; we have, therefore, been unable to illustrate the theory as amply as we could have wished. We hope, however, that we have been successful in the endeavour to avoid sacrificing clearness to brevity.

We have made no attempt to give anything like a complete list of the experiments that may be performed with the apparatus that is at the present day regarded as the ordinary equipment of a Physical Laboratory. We have selected a few-in our judgment the most typical-experiments in each subject, and our aim has been to enable the student to make use of his practical work to obtain a clearer and more real insight into the principles of the subjects. With but few exceptions, the experiments selected are of an elementary character; they include those which have formed for the past three years our course of practical physics for the students preparing for the first part of the Natural Sciences Tripos; to these we have now added some experiments on acoustics, on the measurement of wave-lengths,

and on polarisation and colours. Most of the students have found it possible to acquire familiarity with the contents of such a course during a period of instruction lasting over two academical terms.

The manner in which the subjects are divided requires perhaps a word of explanation. In conducting a class including a large number of students, it is essential that a teacher should know how many different students he can accommodate at once. This is evidently determined by the number of independent groups of apparatus which the Laboratory can furnish. It is, of course, not unusual for an instrument, such as a spectrometer, an optical bench, or Wheatstone bridge, to be capable of arrangement for working a considerable number of different experiments; but this is evidently of no assistance when the simultaneous accommodation of a number of students is aimed at. For practical teaching purposes, therefore, it is an obvious advantage to divide the subject with direct reference to the apparatus required for performing the different experiments. We have endeavoured to carry out this idea by dividing the chapters into what, for want of a more suitable name, we have called 'sections,' which are numbered continuously throughout the book, and are indicated by black type headings. Each section requires a certain group of apparatus, and the teacher knows that that apparatus is not further available when he has assigned the section to a particular student. The different experiments for which the same apparatus can be employed are grouped together in the same section, and indicated by italic headings.

The proof-sheets of the book have been in use during the past year, in the place of the original MS. books, in the following manner :—The sheets, divided into the sections

above mentioned, have been pasted into MS. books, the remaining pages being available for entering the results obtained by the students. The apparatus referred to in each book is grouped together on one of the several tables in one large room. The students are generally arranged in pairs, and before each day's work the demonstrator in charge assigns to each pair of students one experiment—that is, one section of the book. A list shewing the names of the students and the experiment assigned to each is hung up in the Laboratory, so that each member of the class can know the section at which he is to work. He is then set before the necessary apparatus with the MS. book to assist him ; if he meets with any difficulty it is explained by the demonstrator in charge. The results are entered in the books in the form indicated for the several experiments. After the class is over the books are collected and the entries examined by the demonstrators. If the results and working are correct a new section is assigned to the student for the next time; if they are not so, a note of the fact is made in the class list, and the student's attention called to it, and, if necessary, he repeats the experiment. The list of sections assigned to the different students is now completed early in the day before that on which the class meets, and it is hoped that the publication of the description of the experiment will enable the student to make himself acquainted beforehand with the details of his day's work,

Adopting this plan, we have found that two demonstrators can efficiently manage two classes on the same day, one in the morning, the other in the afternoon, each containing from twenty-five to thirty students. The students have hitherto been usually grouped in pairs, in consequence of the want of space and apparatus. Although this plan

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