We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
400 OPTICAL PROJECTION spark being more clearly seen than most eyes can discern it in the brilliant spark itself. To project the oscillating character of the discharge, as pointed out by Dr. 0. J. Lodge in his own demonstrations, we require a battery of considerable size or capacity, and must introduce into the circuit a number of coils of wire. A quarter-inch spark is a good distance. Having at hand several large coils of wire, and introducing one after the other into the circuit, the spark will be found to give at each addition a lower note ; and when this reaches a certain low- ness of pitch, it is broken up if focussed on the screen and then reflected from a rotating mirror. A better apparatus, however (since the breaking-up of the spark can only be seen at one precise moment), is to project it through a number of lenses of the same focus ranged round the edge of a rotating disc, at slightly different radial distances, as devised by Mr. C. V. Boys. By this means the effect cannot fail to be pro- jected upon the screen, and there is less loss of light also than when the mirror is employed. The heating effect of the discharge is effectively shown by a simple form of air thermometer employed by Prof. Weinhold. Through a dry cork (which is sufficient insulation) in a small bottle are passed wire terminals, connected by a rather narrow strip of tinfoil in the shape of a U« A bent glass tube also passing through the cork is drawn out into a small bore, and then the bottle being slightly warmed and the tube dipped in coloured liquid, as the air cools it draws in a small portion which serves as an index. Projecting this, on making dis- charge through the foil, the air in the bottle is sufficiently heated to move the drop of fluid conspicuously upon the screen. 240. Current Electricity.—Galvanometers.—In the vast majority of experiments, the existence or calling into action of a current will have to be demonstrated by projecting the movement of a galvanometer, which can be easily done in various ways.