. Des expériences de laboratoire physique : mécanique, l'optique et de la chaleur. n statique ou bobine machinewhen transmises entre les électrodes de l'autre métal étudié.Afin d'obtenir suffisamment d'éclat, une grande capacité d'accueil doit être connecté en parallèle entre les bornes d'étincelles. C'est idéalement la forme de jarres Leyden. En plus des lignes lumineuses en raison de l'volatilizedfrom les électrodes métalliques, l'étincelle aussi manyair spectre contient des lignes en raison de le gaz chauffé par lequel le dischargetakes place. Ils peuvent, par l'introduction d'sufficientinductance dans le circuit, l'être en grande partie elim

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. Physical laboratory experiments : mechanics, optics and heat. n coil or static machinewhen passed between electrodes of the metal to be studied.In order to obtain sufficient brilliancy, a large capacity shouldbe connected in parallel across the sparking terminals. Thisis conveniently provided in the form of Leyden jars. In addition to the bright lines due to the metal volatilizedfrom the electrodes, the spark spectrum also contains manyair lines due to the heated gas through which the dischargetakes place. These can, by the introduction of sufficientinductance in the circuit, be largely eliminated. In studyingarc and spark spectra, the arc or spark should not be placedclose to the slit, but at some distance from it, and the imageof the light source formed on the slit by means of a lens.This has the advantage that the spectrum of different partsof the light source can be studied. Vacuum Tube Spectra.—Gases and vapors at ordinary tem-peratures are rendered luminous for spectrum observationsby enclosing them in a suitable form of vacuum tube, and. I r passing an electric discharge through the tube. The formof vacuum tube best adapted for this purpose is shown inFigure 7, (a) and (6). These are so-called end on tubes, thedischarge being viewed along instead of across the capillary, as in the ordinary form of Geissler tube, Figure 7 (c). 24 PHYSICAL LABORATORY EXPERIMENTS The intensity of the light source is thus greatly increased.It is not necessary to introduce a capacity in the circuit inparallel with these tubes, as in the case of the spark dis-charge. After a time, especially if driven hard, the vacuumgoes up in most Geissler tubes, owing to the absorption ofthe gas by the electrodes. The discharge then passes withgreater difficulty, and the Geissler effect gradually disap-pears, giving place, when a Crookes vacuum is reached, to cathode rays, as shown by the phosphorescent effectwhich they excite on the glass. Such tubes are unsuited forspectrum work. Spectrum An

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