CATHODE RAYS are the streams of negatively charged particles leaving the cathode in a discharge tube containing a gas at a low pressure (see ELECTRICITY, CONDUCTION OF : Gases) . The cathode rays consist of electrons (see ELECTRON) . Cathode rays have many applications, one of the chief being the excitation of X-rays by the impinging of swift electrons against a hard anticathode (see also COOLIDGE TUBE; X-RAYS ; and also SPECTROSCOPY : X-Ray). This bombardment, besides exciting X-rays, generates a considerable amount of heat and the anti cathode can be used as a cathode ray furnace for melting small quantities of metal, etc. Forms of apparatus making use of the deflection of a beam of cathode rays by magnetic and electric fields are the cathode ray oscillograph, or cathode ray tube, which indicates the variation and values of an alternating current or voltage (see INSTRUMENTS, ELECTRICAL), and the cathode ray manometer, in which a change of pressure is communicated to tourmaline crystals which become electrically charged (see ELECTRICITY) and produce an electric field which is measured by a cathode ray oscillograph (see BALLISTICS).