Popular Science Monthly/Volume 43/July 1893/Teaching Physics
By Prof. FREDERICK GUTHRIE, F. R. S.
THERE is no physical science without exactness, and there is no exactness without measurement. Far as we are still from understanding the mystery of life, it is not to be denied that the greatest advances in biology have been due to exactness in observation and quantitative comparison. This is more markedly the case with the sciences of geology and astronomy. Still more is this to be insisted on in the study of the forces of inanimate Nature. I have always, for instance, tried to persuade those of my friends who are engaged in teaching chemistry that they would do well to begin at once with quantitative methods and determinations in the laboratory, synthetic as well as analytic.
This quantitative element is still more essential in physics. There everything should be quantitative and exact. But there are different degrees of exactness. No one would expect from the average student of chemistry that all his analyses should be of the same degree of refinement as though he were determining the atomic weight of an element. Let his analyses be sufficiently exact to convince him of the faithfulness of Nature and the trustworthiness of the statements of the science.
Now, in bringing before you to-night a short account of the system of teaching practical or laboratory physics which has been adopted at the Government Science Schools with which I am connected, I must speak a few words as to the origin of that system.
The problem was briefly this. Given a class of students of various ages, from sixteen to sixty, and of various degrees of general knowledge and ability. Assume that they are all anxious to learn, and that none of them have worked systematically before in a physical laboratory, and let the instruction be limited to a few months—say four.
The problem is to give them a sound but necessarily elementary training in the science, so that all shall have an opportunity of acquiring such a knowledge of physics as no educated man should be without, and no scientific man dare to be without, and to those who have the ability, the opportunity, and the desire, a trustworthy foundation on which to base their further studies.
The scheme almost necessarily formed itself into the following: The student attends a lecture every morning, except Saturday, at ten o'clock. These lectures, in the present case, are about seventy in number. At eleven o'clock he goes into the laboratory, provided with a few tools; there he finds the necessary material for making apparatus relating to the lecture. He has also printed instructions directing him how to make and how to use the apparatus when made. He finds also working models of such apparatus for his guidance. These instructions he carries out under the supervision and advice of a skilled assistant.
The instruments the student of average skill can and does make under proper instruction with these means are far more accurate than those he is at all likely to be able to buy. I do not say that his divided circles will be as accurate as those of Troughton and Sims, nor will his spectroscope compare with one of Hilger's, nor his resistance coils with those of Elliott, nor his barometer with the one at Kew; but I do say that his barometer is a far more exact instrument than one for which he would have to give several pounds; that his spectroscope will divide the sodium line; that his coils are true to the thousandth of their nominal value; that he can determine the wave-length of light to within 1, of the truth, the specific heat of a metal to 1, and the length of a sound-wave to 1 of the truth. The only bought instrument of precision which the student uses in the elementary course is the balance. He has generally, however, acquired some skill with this, and in the manipulation of glass, in the chemical laboratory.
Starting with a tuning fork which is given to him, and the monochord which he makes, the student is able to verify the intervals of the gamut as dependent on length of string. He then examines the effects of variation of diameter, of tension, and of weight of the string.
Tuning forks are, however, seldom exact. The actual pitch of the fork is found by the method of sinuosities. A smoked glass plate is dropped in front of a style on the fork, and so the fork writes its own number. Hence, by means of the length of the resonant cavity, the velocity of sound in air is obtained with some accuracy, and by the method of longitudinal vibrations the velocity in wood, glass, and brass, etc., follows. The rule of the transverse vibrations of rods is examined. The production of harmonics on strings, rods, and in tubes is shown, and a number of experiments follow concerning the velocity of sound in different gases as determined by dust figures.
Having made and graduated both a direct alcohol and a differential air thermometer, the absolute expansions of water and alcohol are determined. Very accurate results may easily be got as to the latent heats of water and steam. Then the student, having made his calorimeter, determines the specific heats of iron, copper, zinc, tin, and lead. The specific heats of a few liquids are determined either by direct comparison with water or indirectly with the metals.
In light, the chief work consists of the following: The making and use of the diaphanous and shadow photometers; the making of an instrument for examining the rules of reflection and refraction, and the verification of these rules; the determination of refractive indices of liquids and their dispersive powers; the images from curved mirrors, the measurement of focal lengths, and the curvative and refractive indices of lenses. A few experiments concerning' plane polarized light are followed by the determination of the wave-length by a grating, and the construction and use of the spectroscope.
The principal pieces of apparatus constructed for work in electricity are: A gold leaf electroscope; a differential condenser; a sand-dropping accumulator; a Leyden jar; an electrophorus; a dry pile; a voltaic cell; a differential galvanometer; a resistance bridge; a set of resistance coils; a tangent galvanometer; a potentiometer; a thermo-element; a thermopile. And by these apparatus typical experiments and measurements, of which the following are a few, are made: The study of magnetic curves; the action of the current on the needle; the relation between length, weight, and resistance in wires; the effect of temperature on resistance; the law of divided circuits; specific resistance; electromotive force; internal resistance of cells, and so on.
Electricity, especially voltaic, lends itself perhaps more abundantly to exact measurements in the elementary laboratory than the other branches, and it is on this account, and because it is the last subject treated of, and so claims any spare time at the end of the term, that it occupies a rather prominent part. I do not hold that it has really any greater educational value than the other branches, and certainly in a general educational course it is not for me to give it prominence, because just now it has a considerable technical development. I trust the time may never come when any branch of physics will be considered as of comparatively little importance in general education.
To-day I have particularized the method of teaching one branch of science. I have had to use strong language, for I feel strongly, and I have been addressing strong people. Of this, at least, you and all men may be well assured, that I will not cease to proclaim, as long as strength is given to me, that the hope of science is the hope of the world; that while I yield to none in my love of imagination, of literature, and of all the fine arts, they are as the gracious flowers of the mind-plant whose leaves and roots are the truths of science. True that the living plant is most beautiful when it is in blossom. He who plucks off the flower, while marring the beauty of the plant, destroys the fruit forever.—Abridged from the Journal of the Society of Arts.