Apeil 10, 1896.] 



SCIENCE. 



539 



and the capacity adjusted carefiiUj' until 

 the break-spark was a minimum and gave a 

 sharp cracking sound. Too much capacity 

 will not necessarily increase the sparking, 

 but it will diminish the inductive effect 

 which is noticed immediately in the dimin- 

 ished intensity of the discharge. A power- 

 ful coil with a smoothly working rotary in- 

 terrupter will be found a most satisfactory 

 apparatus in experiments with Routgen ra- 

 diance. 



Edison^s Fluoroscope. A fluorescent 

 screen, made by Aylsworth & Jackson, of 

 East Orange, N. J., according to Mr. Edi- 

 son's directions, will be found an indis- 

 pensable aid in these experiments. The 

 salt employed is tungstate of calcium and 

 it is so powerful that with a satisfactorily 

 working tube it will show a noticeable 

 fluorescence at a distance of over thirty feet. 

 Those who have struggled with barium- 

 platino-cyanide screens will appreciate fully 

 Mr. Edison's improvement. This fluores- 

 cent screen was employed successfully for 

 three distinct purposes. First, to study the 

 operation of the vacuum tube under vari- 

 ous conditions; secondly, to shorten the 

 time of exposure in photography; and 

 thirdly, to study the phenomena of diffuse 

 reflection. 



The most Efficient Working of the Tube — 

 The Critical Temperature. The tubes 

 employed were an old pear-shaped Crookes 

 tube with a cross and several pear-shaped 

 German tubes, imported sometime ago by 

 Eimer & Amend, of ISTew York. They all 

 had discs at each electrode. Very satis- 

 factory tubes are also being made now at 

 the lamp works of the General Electric 

 Company at Harrison, New Jersey. These 

 were also employed in my experiments with 

 completely satisfactory results. No fresh 

 tube works quite satisfactorily with a power- 

 ful coil and a rapid rate of interruption ; 

 it heats too much, and the vacuum becomes 

 thus rapidl}' impaired and the intensity of 



the Eontgen radiance is very much dimin- 

 ished. This is true even of larger tubes. 

 Each new tube must undergo first an elec- 

 tric treatment. I have described this mat- 

 ter at some length at the meeting of the 

 Academy on March 2d. Since that time I 

 have investigated it more fully and brought 

 it to a satisfactory termination. Mr. Tesla 

 has also in the meantime discussed this 

 matter, but in what appears to me to be a 

 somewhat fanciful way. He imagines that 

 the vacuum of a Crookes tube becomes more 

 and more attenuated by the passage of cur- 

 rent through it on account of the expulsion 

 of the gas through the walls of the bulb. 

 He maintains that he even succeeded in 

 piercing electrically a small hole in the tube 

 through which the gas from the vacuum 

 was exjjelled with so enormous a velocity as 

 to prevent the outside air from rushing in. 

 This marvelous experiment does certainly 

 support Mr. Tesla's favorite molecular bom- 

 bardment theory, but it seems to leave us 

 with the gloomy propect of having to refill 

 our tubes from time to time with a fresh 

 vacuum. The following experiments, how- 

 ever, lead to the conclusion that this neces- 

 sity will probably never arise and that Mr. 

 Tesla's interpretation of the cause of vari- 

 ation of the vacuum during the discharge is 

 probably wrong. The electrical treatment 

 of the tube is simply this : Pass a sufficiently 

 strong current until the tube becomes so 

 hot as to lose much of its RSntgen radiance. 

 Stop then and let it cool. Eepeat the oper- 

 ation and observe that after each operation 

 the vacuum has gone up and that the R5nt- 

 gen effect becomes stronger. It is not ad- 

 visable to drive the vacuum much beyond 

 the sparking distance between the electrodes 

 on the outside. But even if this point has 

 been reached then a judicious application 

 of the Bunsen flame to the tube will enable 

 the coil to force and maintain a strong 

 enough current through the tube so as to 

 heat it gradually', which increases the facil- 



