X-Rays from Boron and Carbon. 149 



gauze on positive ions and electrons is indicated in ap- 

 paratus II., fig. 1.) The only way, then, for the metal 

 plate Z to acquire a positive charge would be for the few 

 positive ions turned back on R to release electrons here, and 

 for these electrons, by virtue of R being 18 volts negative 

 to Z, to travel to Z. This, however, would give Z a 

 negative charge. To account for a spurious positive charge 

 simulating the photo-electric effect sought, one would have 

 to assume that each electron hitting Z released more than 

 one electron on impact. The work of v. Baeyer, and still 

 more the recent work of Millikan and Barber *, shows that 

 these 18-volt electrons could only release a much smaller 

 number of electrons than the original number of 18-volt 

 electrons impinging on Z. According to Millikan and 

 Barber, electrons must have energies of 200 volts or more 

 before the secondary electrons exceed the primary in 

 numbers. This result is for copper; presumably something 

 of the same order holds for nickel and silver. One may 

 safely conclude that the positive current from the plate Z is 

 a pure photo-electric current due to radiation f . 



The boron or carbon target was subjected to a preliminary 

 heat treatment, lasting several hours, in a silica tube heated 

 to redness and exhausted by a diffusion-pump. The tung- 

 sten filament, ready mounted in its glass tube, was heated to 

 a white heat in an auxiliary vacuum for some time. The 

 target was mounted 1 mm. away from the filament, and the 

 glass tube carrying the target and filament so assembled 

 was quickly sealed into the main tube, which was then ex- 

 hausted by a diffusion-pump. An electric oven was built up 

 around the apparatus and heated to 500° C. with the pumps 

 running continuously. After several hours' heating, the 

 pressure fell to about '00002 mm. The heating was then 

 continued at 380° C, during which treatment no pressure 



* Millikan and Barber, Proc. Nat. Acad. Sci. vii. p. 18 (1921). 



t To get an idea of the effect of allowing positive ions to get 

 through, the potential of Q was reduced to zero. With 45 volts 

 accelerating the electrons from F to T (i. e., F zero, T +45 volts), the 

 positive ion effect plus the radiation effect at Z (Q, zero) was only about 

 four times the radiation effect alone (Q, +600 volts). With higher 

 accelerating voltages the ratio was decreased, as the radiation increased 

 rapidly with the energy of the bombarding electrons. It is unlikely 

 that these positive ions, when turned back on the upper side of R, would 

 release anything like as many electrons as there were positive ions, and, 

 in any case, we have already seen that these 18-volt electrons from R 

 impinging on Z could not account for the flow of electrons from Z. 

 Incidentally, it may be mentioned that when the potential of Q was zero 

 permitting positive ions to get to Z, the positive ion effect was of the 

 order of 10~ 9 of the electron stream from F to T. 



