1027 



measured with an eleclrometei according to Wulf '), whicli conld 

 be used up to 40000 Volts. It was calibrated with a compensation- 

 apparatus according to Brooks. The ratio between the various 

 measuring ranges was checked witli a sparking-gap between 

 needle-points. 



The condensers were glass tubes 1 metre in length and 8 cms in 

 diameter which were silvered both on the inside and outside. At 

 the ends and at the bottom the glass was locally thickened in order 

 to pre\ ent the spark piercing the glass '). They were blown for us 

 in Philips' glass-factory ''). Their capacity was about 0,005 of a 

 micro-farad. When rectified alternating current was used (see below), 

 an additional condenser of 0,007 m.F. had to be connected up in parallel 

 in order to obtain a sufficiently constant potential. With the higher 

 potentials the condensers were charged with a Wimshurst machine. 

 In order to make the charging take place very slowly, a discharge- 

 tube with pointed electrodes was put in parallel to the machine. 

 The air-pressure in this tube could be varied. During the measure- 

 ments air was slowly admitted. Hereby the tension at the pointed 

 electrodes was gradually raised. As soon as the sparking potential 

 in the discharge-tube which is filled with the mixture is reached and 

 passed, the discharge takes place and the potential falls to a smaller 

 value. The highest reading of the electrometer was taken. 



With the smaller tensions this arrangement was not found 

 satisfactory. For this reason we used in this case for charging the 

 condensers a 7000 volt-transformer with rectifiers joined \\\) as 

 described by Greinacher^). Instead of cells of the GRAETZ-pattern 

 we used thermo-ionic rectifiers of our own manufacture, so called 

 kenotrons ^). By a regulation of the heating current any charging- 

 speed desired could be obtained. 



In all our experiments we have taken care that electrostatic 

 disturbances were kept down as low as possible. 



3. Measurements. The apparatus were first checked by a series 

 of determinations of the sparking-potentials in air. The air was freed 

 of carbon dioxide and water vapour. 



1) Th Wulf, Phys. Zeitschr. (11), 1090, 1910. 



2) J. MÓSCICKI. E. T. Z. 1904, 527. 



3) Our thanks are due to Mr. P. J. Sghoonenberg. who superintended the 

 manufacture of the condensers. 



*) H. Greinachee, Phys. Zeitschr. (15), 412, 1914. 

 5) J. LanGxMuir, Electrician 1915. LXXV. 240. 

 A. W. Hull, Electrician 1916. LXXVII, 220. 



