. Radiography and the 'X' rays in practice and theory : with constructional manipulatory details . sely as the pressure between 11 in. of mercuryand 30 in. (which is equal to the pressure of one atmos-phere). At lower pressures a greater difference ofpotential must be used to produce a spark. When wecome to an exhaustion equal to about tuu^ott of anatmosphere, required to produce a dischargeis very nearly half as much again as that requiredunder ordinary atmospheric conditions. § 7. When such high vacua are reached, the electricdischarge produces httle or no visible light; but itspre
. Radiography and the 'X' rays in practice and theory : with constructional manipulatory details . sely as the pressure between 11 in. of mercuryand 30 in. (which is equal to the pressure of one atmos-phere). At lower pressures a greater difference ofpotential must be used to produce a spark. When wecome to an exhaustion equal to about tuu^ott of anatmosphere, required to produce a dischargeis very nearly half as much again as that requiredunder ordinary atmospheric conditions. § 7. When such high vacua are reached, the electricdischarge produces httle or no visible light; but itspresence is capable of being rendered evident by itspower of causing certain bodies to glow with light, or fluoresce. If the apparatus take the form of a glass 16 hAhTOGRAPHY tuU ov Imll) isio li^. 2) with a Ilaliiiuiii electrodeat each end, as the glass itself is a body capable of* fluorescing;, the interior of the bulb is seen to glowwitli a soft yellowish light, which may vary accordingto the degree of vacuum in the tube and the voltage ofthe current, passing from a pale apple-green to a dis-. Fio. 2.— Crookes Tube (Oli» Form). tinct canary-yellow colour. In any case, however, thelight is so feeble as to be barely perceptible in day-light. § 8. Whatever may be the nature of the vibration Usually designated a Crookes tube, from the name of thescientist who first investigated the properties of these high vacua. VACUUM ON ELECTRICAL DISCHARGES 17 or undulation set up in the highly rarefied mediumcontained in the exhausted tube, what is certain is, thatit possesses properties which partake at once of those oflight and of electricity, while differing in many respectsfrom both. One of the properties common to electricityis that of setting up inductive effects in surroundingmedia, and of attracting light bodies placed in thevicinity of the exhausted tube wherein the discharge istaking place. These effects may be easily shown if a tube
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