90 Discharge of Negative Ions by Glowing Metallic Oxides. 



to send a current through the tube. But if the polarity were 

 reversed, so as to make the glowing oxide electrode the anode, 

 and the cold metal electrode the cathode, then at low pressures 

 several thousand volts would be necessary to send a current 

 through the tube. Hence, if such a discharge-tube be in- 

 troduced into a low-voltage (110 to 220 volt) alternating- 

 current circuit, it will act as a valve, allowing only one half- 

 wave of the alternating current to pass through it. The tube 

 may then be used in a manner similar to that of Giiitz's 

 aluminium rectifier, for the purpose of charging accumu- 

 lators, or working induction-coils having electrolytic inter- 

 rupters, by means of alternating currents. 



3. If the limiting value of the current be exceeded in a 

 discharge-tube with glowing oxide cathode, the cathode drop 

 may be given any desired value, and so the cathode rays any 

 desired velocity. 



By the use of a cathode consisting of a narrow bare strip 

 of platinum foil, having only a small circular patch (about 

 1 mm. 2 in area) of CaO on it, the entire discharge, when the 

 platinum glows, may be made to pass through the CaO patch, 

 so that from this latter there issues a very thin pencil of 

 cathode rays, having a direction normal to the foil, These 

 rays may be given any desired velocity by a proper choice of 

 the temperature and the current. They are extraordinarily 

 bright, so as to be visible even in a fairly large darkened 

 lecture-room. 



If the tube be introduced into a uniform magnetic field 

 whose direction is perpendicular to that of the cathode rays 

 as they leave the cathode, the rays are bent into completely 

 closed circles, whose radius of curvature is easily determined. 

 If in addition to this the total discharge potential-difference 

 be measured by means of a voltmeter, and from it there be 

 subtracted 20 volts to allow for the anode drop, the cathode 

 drop becomes approximately known, and thus it becomes 

 possible to demonstrate to a large audience, in a simple and 

 easily intelligible manner, the determination of the ratio of 



charge to mass - and of the velocity [y] of cathode rays. 



By varying the cathode drop within wide limits (5 to 200 

 volts or more) it then further becomes possible to demonstrate 



that the ratio - remains constant within these limits, while 



v, on the other hand, increases with increasing cathode drop. 



Erlangen, Physical Institute of the University, 

 April 1905. 



