Page images
PDF
EPUB

tiously worked out are calculated to cause the experimenter to think and reason for himself.

Following the series of tests is an Appendix, containing the Algebraical solutions of the various formulæ met with in the tests, and these the student is strongly urged not to refer to until he has tried, by all the means in his power, to solve the inference for himself.

The Appendix also contains complete descriptions and illustrations of a large amount of the apparatus which may be employed in carrying out the tests, a considerable proportion of it being such as will be found in almost every college and testing-room.

Useful constants and tables which are frequently needed in electrical engineering tests are added at the end of the book. A good deal of the apparatus illustrated has been constructed by the mechanical assistants, Messrs. John Watkinson and Herbert Addy, of the Physical and Electrical Engineering Departments respectively of the College.

In conclusion, I wish to express my sincere thanks to my valued friend, Mr. Charles Mercer, M.A., for the very considerable amount of trouble he has taken in producing many of the photographs from which some of the illustrations are obtained; to Dr. John Henderson, for permission to use the tables of squares and reciprocals of numbers; to Messrs. Kelvin & James White, for permission to use the tables of doubled square roots; to His Majesty's Stationery Office, for allowing me to use the tables of logarithms and anti-logarithms; to Messrs. Longmans, Green & Co., for their kindness in lending eight illustrations and a little printed matter from Practical Electrical Testing; to Mr. Herbert Addy, for the trouble he has taken in making the drawings from which the illustrations of joints, made in electric light cables, are taken, and also for reading through the proofs simultaneously with myself; and further to Messrs. Nalder Bros. & Co., Kelvin & James White, Siemens Bros. & Co., Crompton & Co., and Evershed & Vignoles, for their kindness in lending me the blocks of some of the illustrations of the very excellent apparatus and appliances made by them.

G. D. A. P. Yorkshire College, Leeds,

1902.

ELECTRICAL ENGINEERING TESTING

(1) Curve Plotting. Introduction.—The practice of recording the results of any ineasurements or tests graphically, as well as in tabular form, in cases where this is possible, cannot be too strongly urged, and is a most important, as well as, in many cases, an indispensable operation. More especially is this the case with a large number of physical measurements, and particularly so with a majority of tests in Electrical Engineering. The practice of curve plotting, as these graphical representations may otherwise be termed, presents the following important features-

(a) It enables the nature of the variation of one quantity with another to be seen at a glance much more clearly than is possible by aid of a table of results.

(6) It enables errors in experimental observations, of which there are sure to be some, to be corrected comparatively easily, which in a majority of cases would be impossible from the table of results.

(c) In the case of the calibration of instruments it enables the law of that under test to be readily observed.

(d) It has the enormous advantage of enabling any intermediate value between those actually observed and tabulated, to be at once obtained accurately. This, it will readily be conceded, is the most important and valuable feature of all, and the ease, as well as the rapidity with which the operation of obtaining intermediate values can be accomplished, will be dependent on the scales chosen in originally plotting the curve in question.

B

It may therefore be profitable to indicate the mode of procedure in plotting curves, and with a view to exemplifying it, the results of a particular test are given in Table I., and the corresponding curve or graphical representation in Fig. 1. They relate to the determination of the Brake Horse Power (B.H.P.) of an electromotor and the corresponding value of its efficiency at each load.

[graphic][subsumed][subsumed][merged small][subsumed][merged small][merged small][subsumed][subsumed][subsumed][ocr errors][subsumed][merged small][merged small][subsumed][subsumed][merged small][subsumed][subsumed][subsumed][merged small]

In testing work generally at least six or eight different determinations throughout the range should be made, where possible, in order that the curve may be drawn more accurately. In most cases a curve constructed on three or four points only would be practically useless and could not be depended on.

Directions for Plotting.—(1) Assuming that all the results have been worked out numerically and entered up in tabular form, the first thing to note is what two sets of quantities have to be plotted together, and secondly, the largest value of each set, for beyond this the scale need not extend.

(2) The left-hand vertical and bottom horizontal sides OP and OQ respectively of the squared sheet of curve paper are termed the axes and are rectangular. They intersect in a point 0 called

« PreviousContinue »