. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. ment is more difficult, but has lately used a pendulum electroscope in a form thatmakes a most sensitive electro-meter. He suspends a giltpith-ball I cm. radius (built upof small pieces cemented) adistance of 364 cm. from theceiling, by two silk fibresfastened 52 cm. apart andmeeting at the ball, behindwhich a scale is arranged. Abrass ball of the same size ismounted on a glass rod, var-nished while warm withparaffin wax. A force of onlyone dyne acting on the sus-pended ball defle
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. ment is more difficult, but has lately used a pendulum electroscope in a form thatmakes a most sensitive electro-meter. He suspends a giltpith-ball I cm. radius (built upof small pieces cemented) adistance of 364 cm. from theceiling, by two silk fibresfastened 52 cm. apart andmeeting at the ball, behindwhich a scale is arranged. Abrass ball of the same size ismounted on a glass rod, var-nished while warm withparaffin wax. A force of onlyone dyne acting on the sus-pended ball deflects it 133mm.; and charging both ballsin contact so as to give thesame charge to each, thecharge c on either was foundin absolute electrostatic unitsby the formula IT 1-8 where d is the distance incm. between the centres ofthe balls, and d the deflectionfrom the vertical in d is over 5j cm. thelaw of inverse squares wasdemonstrated within 1 percent; and by using a proof-plane, Coulombs law of distribution on the parts of a cylin-drical body was verified with close approximation. Such an C=D. Pig. 217 398 OPTICAL PROJECTION apparatus is in fact too sensitive for most lecture purposes;but diminishing this by using shorter fibres, and adjusting ahorizontal strip of glass behind the ball, divided as a scale,many useful quantitative experiments may be made in a verysimple and easy manner. The mere fact that the density of a charge is greater atthe ends of a conductor than in the middle may be shown byProf. Weinholds very simple apparatus (fig. 217). Fourpieces of paste-board with rounded sides and edges are coveredwith tinfoil, and fastened into a box-like shape by strips ofribbon also covered with foil, or by thick foil itself. To themiddle of one angle are suspended by conducting threads apair of small pith-balls ; and close to the middle of the oppo-site angle are fastened two sticks of sealing-wax as insulatinghandles. The whole apparatus is shown at a. If bothhandles ar
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