Fig. 216.
Fig. 217.
154. Disruptive Discharges. We have seen, in the study of induction coils, that a spark can jump several inches between the terminals of the secondary coil. The attraction between the two oppositely charged terminals gets so great that it overcomes the resistance of the air-space between them, a brilliant spark passes, and they are discharged. This sudden discharge is said to be disruptive, and it is accompanied by a flash of light and a loud report. The path of the discharge may be nearly straight, or crooked, depending upon the nature of the material in the gap between the terminals.
Fig. 218.
Fig. 219.
155. Effect of Air Pressure on Spark. The disruptive spark takes place in air at ordinary pressures. The nature of the spark is greatly changed when the pressure of the air decreases. Fig. 216 shows an air-tight glass tube so arranged that the air can be slowly removed with an air-pump. The upper rod shown can be raised or lowered to increase the distance between it and the lower rod, these acting as the terminals of an induction coil. Before exhausting any air, the spark will jump a small distance between the rods and act as in open air. As soon as a small amount of air is removed, a change takes place. The spark is not so intense and has no definite path, there being a general glow throughout the tube. As the air pressure becomes still less, the glow becomes brighter, until the entire tube is full of purple light that is able to pass the entire length of it; that is, the discharge takes place better in rarefied air than it does in ordinary air.
156. Vacuum-Tubes. As electricity passes through rarefied gases much easier than through ordinary air, regular tubes, called vacuum-tubes, are made for such study. Fig. 217 shows a plain tube of this kind, platinum terminals being fused in the glass for connections. These tubes are often made in complicated forms, Fig. 218, with colored glass, and are called Geissler tubes. They are often made in such a way that the electrodes are in the shape of discs, etc., and are called Crookes tubes, Fig. 219. A slight amount of gas is left in the tubes.