Page:Popular Science Monthly Volume 43.djvu/404

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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