Vacuum-tubes

Vacuum-tubes, glass tubes in which a 'Vacuum' (q.v.) has been made and which have then been hermetically sealed, and into the opposite extremities of which platinum wires have been soldered, with an arrangement at the free ends of these whereby they may be connected with the secondary wires of an induction coil, or may, generally, be put into the circuit of an electric current (see ELECTRICITY). The object of this arrangement ('Geissler's tubes') is to pass a high-pressure current of electricity through the so-called vacuum, which is in reality a highly rarefied quantity of the particular gas (air, oxygen, nitrogen, &c.) with which the tube had been filled prior to exhaustion. When such a current passes, the residual gas glows with a bright light the colour of which varies with the nature of the gas in the tube, the glow being brighter round the negative electrode, but being separated from it by a thin dark layer. The discharge is repelled, attracted, or made to turn round a magnet brought near it in the same way as a perfectly flexible current-bearing conductor would be. If any fluorescent substances, such as a solution of sulphate of quinine, or uranium glass, be placed round the rarefied gas, these will glow brightly with their own fluorescence-colours, under the influence of the ultra-violet rays, in which the light of the discharge is rich. If a part of the tube be narrow the glow is broken up into discs or striæ, the cause of which is not clear. These striæ can in some cases be rendered less numerous and at a greater distance from one another by slowing the frequency of oscillatory discharge of each make-and-break of the induction coil. This can be done by interposing in the circuit a coil of numerous convolutions. The striæ are, however, observed when the current is apparently continuous, as when the source of electricity is a battery of 500 or more Daniell's cells. Gassiot inferred that the discharge of the battery itself through a rarefied gas is not continuous but intermittent. The number and position of the striæ is altered by altering the resistance in the circuit. If the discharge is rapidly and regularly intermittent the glow is sensitive to the approach of a conductor connected with the earth or a large condenser, and is repelled or attracted thereby according to the arrangement of the apparatus. It is not necessary that the platinum terminals should be in contact with the wires of an electric circuit; it is sufficient to put them to widely different potentials, as by lowering the tube into the electric field between the knobs of a Holtz machine: the residual gas then glows without contact. The same thing occurs when the field is one of high potential and rapid alternation. If the vacuum be reduced to $\backslash frac\{1\}\{1000000\}$th of an atmosphere the current will not pass and there is no glow. At about one-millionth of an atmosphere the molecules become so few that there are very few collisions between them. There is no light produced in the body of the gas unless these collisions occur. If the tube be less in its dimensions than the mean free path (see GAS) of the molecules the first collision of any given molecule leaving the negative electrode will probably be against the glass of the tube itself. The glass accordingly begins to glow with a bright phosphorescent light, but only at such points as can be reached by molecules leaving the negative electrode at right angles and travelling in straight lines. If the negative electrode be so formed as to concentrate the molecular impact upon a diamond it will shine with a green light equal to that of a candle; a ruby or aluminum oxide, bright red; glass, green. The position of the positive electrode appears to have no bearing on the phenomenon. Since the molecules travel in straight lines, any solid obstacle will cast a shadow, and the molecules exert mechanical force when they strike (see RADIOMETER); and they also produce heat which can, when they are concentrated upon a point, melt glass or even a piece of metallic iridio-platinum. These are phenomena of Radiant Matter or the ultra-gaseous state (Crookes) in which the ordinary properties of gases (see GAS) are profoundly modified by the absence of collisions between the molecules. Lenard, following up Crookes and Herz, discovered that the dark or invisible rays emitted from the vacuum-tubes through an aluminium 'window' would, in a dark box, take photographs; and in 1895 Röntgen (q.v.) of Würzburg found that these dark rays, the so-called Röntgen rays, when passed through the hand or other part of the body would imprint a shadow-picture of the bones on a sensitive photographic plate—a discovery speedily applied in various ways, and utilised in surgery.