Thermionic Currents with Potentials. 559 



Note. 



A peculiar phenomenon was observed with a CaO cathode, 

 ihat for purposes o£ better observation was mounted in a 

 ^lass tube which contained a copper gauze anode. After the 

 wire had been under constant temperature, constant pressure, 

 and zero potential for five or six hours, a set of readings for 

 a current-potential curve was taken. Upon changing the 

 potential to 40 volts a blue glow appeared about the cathode, 

 which was brighter at 400 volts, but which disappeared 

 suddenly without affecting the magnitude of the thermionic 

 current. The potential was switched to zero and then put 

 again at 400. The tube lighted very brilliantly. The wire, 

 which had been at about 900° and uniformly bright, became 

 invisible except for a spot about 2 mm. long near the middle 

 of the wire. This spot was dazzling white, and must have 

 been near 1800° C. The thermionic current was too large 

 for the instrument to record. The anode became heated 

 opposite the glowing spot, and some red sparks passed. The 

 walls of the glass tube glowed with a blue fluorescence except 

 for a patteru corresponding to the shadow of the meshes of 

 the gauze which did not show the fluorescent blue. A lateral 

 tube connecting the experimental chamber with a vertical 

 tube running to the pump happened to be just opposite the 

 bright spot. There was a sharply defined fluorescent spot 

 on the vertical tube opposite the open end of the lateral 

 -connexion, though this wall was 3 centimetres behind the 

 gauze anode and 4 centimetres from the hotwire. The glow 

 continued approximately two minutes and stopped abruptly. 

 Subsequent investigation showed the CaO cathode to be 

 covered with finely divided copper or copper oxide. Similar 

 discharges were obtained later, but never a second time from 

 the same spot on the wire. They were increasingly hard to 

 stimulate, and after a time ceased altogether. It was found 

 that a rest-period was necessary to get even a second discharge. 

 Similar discharges occurred in another apparatus (the brass 

 .apparatus shown in fig. 2) and also with a BaO cathode. 



Palmer Physical Laboratory, 

 Princeton. 

 July 1915. 



