. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. fectly easy to arrange an entire fixed apparatus, and projectit by the vertical method. But the truly scientific demon-strator will rather preferto place the bare needlefirst in the focal plane, andbring over his wire inde-pendently, as in fig. 222,simply because his ex-planation will thus bebetter understood, which ishis object. Amperes experimentsshowing the attraction orrepulsion of parallel cur-rents may be shown byany of the usual apparatusmade on a small scale ; buthere again the same
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. fectly easy to arrange an entire fixed apparatus, and projectit by the vertical method. But the truly scientific demon-strator will rather preferto place the bare needlefirst in the focal plane, andbring over his wire inde-pendently, as in fig. 222,simply because his ex-planation will thus bebetter understood, which ishis object. Amperes experimentsshowing the attraction orrepulsion of parallel cur-rents may be shown byany of the usual apparatusmade on a small scale ; buthere again the same ruleapplies, and it is better toconfine the set apparatusto the movable portion,and bring up the other current independently, as in fig. 223,which is an arrangement by Prof. Forbes. A small arrowmay be fixed to the wire which is .held; and in this way dp2. 404 OPTICAL PROJECTION there is no room for misunderstanding. The arrangementknown as Eogets Spiral gives another very elegant illus-tration of the attraction of parallel currents. The wire carry-ing the current is wound into a vertical helix, suspendedat the top from one terminal, and dipping at the bottom intomercury, which forms the other terminal. The coils shouldbe not less than an inch in diameter, and wound closelyenough to be very easily movable: then on passing the current the coils drawmore closely together,which is very conspicuousupon the screen. If morepowerful action is neededan iron core may be intro-duced, as in the figure, butthis will rarely be neces-sary. Any of the usual ap-paratus for showing therotation of currents, mag-nets, or galvanic cells, iseasily constructed smallf enough for projection, and5 need not be further dweltupon. For many electro-mag-netic experiments it isconvenient to arrange two parallel horizontal brass rods pro-jecting from beneath t
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Keywords: ., bookauthorwrightle, bookcentury1900, bookdecade1900, bookyear1906
