Fig. 208 shows the mechanism by which the carbons are regulated. Fig. 209 shows a form of _double carbon_, or _all-night_ lamp, one set of carbons being first used, the other set being automatically switched in at the proper time. [Illustration: Fig. 215.] Figs. 210, 211 show forms of _short arc lamps_, for use under low ceilings, so common in basements, etc. Fig. 212 shows a _hand-feed focussing_ type of _arc lamp_. In regular street lamps, the upper carbon only is fed by mechanism, as it burns away about twice as fast as the lower one, thus bringing the arc lower and lower. When it is desired to keep the arc at the focus of a reflector, both carbons must be fed. Fig. 213 shows a _theatre arc lamp_, used to throw a strong beam of light from the balcony to the stage. Fig. 214 shows the arc lamp used as a search-light. The reflector throws a powerful beam of light that can be seen for miles; in fact, the light is used for signalling at night. Fig. 215 shows how search-lights are used at night on war-vessels. CHAPTER XXIII. X-RAYS, AND HOW THE BONES OF THE HUMAN BODY ARE PHOTOGRAPHED. [Illustration: Fig. 216.] [Illustration: 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. [Illustration: Fig. 218.] [Illustration: 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.