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. 



