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IV. (hi the Discharge of Negative Electricity from Hot Calcium and from Lime. 



By FRANK HORTON, D.Sc., B.A., Fellow of St. John'n College, and Clerk- 

 Maxicell Student of the University, Camlnidge. 



Communicated by Professor J. J. THOMSON, F.R.S. 







Received December 10, 1906, Read January 31, 1907. 



INTRODUCTION. 



THE discharge of electricity from hot metals has been the subject of a great number 

 of researches by different authors. The metal chiefly used in these experiments has 

 been platinum, on account of its high melting-point, its stability in air, and the ease 

 with which it can be obtained in a state of purity. In the present experiments 

 calcium was chosen for investigation because of its strong electropositive character. 

 Since this implies a great attraction for positive electricity, it would l)e expected that 

 the negative corpuscles would escape more readily from calcium than from platinum. 

 It should, therefore, be possible to obtain a measurable " negative leak " from calcium 

 at a much lower temperature than from platinum or other less electropositive metal. 



The first method of experimenting employed consisted in measuring the saturation 

 current from an electrically heated calcium wire to a surrounding electrode, both 

 being placed in a vacuum, but it was found to be impossible to get a clean surface of 

 calcium in this way, for the metal combines with the oxygen, nitrogen, and water 

 vapour in the air, and becomes more or less covered with a coating of calcium 

 compounds before the apparatus can be fitted up. Another difficulty was soon 

 discovered. When the wire was heated to above a dull red heat, the vapour pressure 

 of the metal was sufficient for it to volatilize and condense on the colder walls of the 

 tube. Thus the wire got thinner at its hottest point, and, consequently, the 

 temperature there rose, and the sublimation increased, until in a few seconds the wire 

 had broken through. 



It was finally decided to make use of the volatility of calcium in order to obtain a 

 clean surface of the metal. The method of experiment was to fix up a platinum strip 

 as the cathode in a vacuum tul)e and to ascertain the manner in which the current 

 from this to the other electrode varied with the temperature of the strip, with the 

 difference of potential between the electrodes, and with the gas pressure in the 



VOL. OOVTI. A 416. 4 - 6 - 07 



