232 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



defined Roentgen ray shadows may be produced. The platinum 

 plate / is usually connected as shown to the aluminum anode a. 

 An interesting feature of the Crookes tube, which is shown in Fig. 

 163, is the small platinum tube t which is sealed through the 

 glass wall. When the vacuum in the Crookes tube becomes too 

 high (presumably by the transformation of the residual gases into 

 non-volatile products), the small tube / is held for a few seconds 

 in the flame of an alcohol lamp and a sufficient amount of hydro- 

 gen passes through the hot platinum to replenish the supply of 

 gas in the Crookes tube. 



131. Conductivity of hot gases and flames. A hot gas is a 



fairly good electrical conductor and this conductivity has been 

 found to be due to the presence of free ions.* The conductivity 

 of a hot gas or flame is shown by the fact that a charged glass 

 rod may be completely discharged by passing the flame of a Bun- 

 sen burner rapidly over its surface. 



132. The electric arc. In order to produce a perceptible dis- 

 charge of electricity (flow of current) through a gas, a very high 

 electromotive force must be used because of the necessity of pro- 

 ducing ionization in the gas by the collision of the moving ions 

 with the gas molecules ; and the amount of current which can be 

 made to flow through a gas is usually very small because of the 

 comparatively small number of these ions. When, however, metal 

 or carbon electrodes are heated to a very high temperature 

 they emit electrons (negative ions) in great numbers f and a 

 very considerable current may then be made to flow through the 

 intervening gas. Thus, a current of an ampere or more may 

 be made to flow between a cold metal anode and a very hot metal 

 cathode in a vacuum tube. When two carbon rods are connected 

 to a battery or dynamo, brought into contact and then separated, 

 the current which begins to flow across the indefinitely small gap 

 between the two carbon rods raises the tips of the carbons to a 



*See J. J. Thomson's Conduction of Eh strict ty Through Gases, pp. 228-249. 

 f See J. J. Thomson's Conduction of Electricity Through Gases, pp. 188-227. 



