﻿Thermions from the Salts of the Alkaline Earths. 127 



Apparatus. 



The apparatus employed in a few of the earlier experiments 

 was that used by Richardson in his experiments upon the 

 salts of the alkali metals. This apparatus was, however, 

 designed for a different purpose, as previously mentioned, 

 and it was soon thought advisable to replace it by another 

 involving the same principles but better suited to the nature 

 of these experiments. In the new apparatus all linear 

 dimensions were increased by a factor three, and the whole 

 so oriented that the normal to the strip pointed directly 

 upward, as in fig. 1. These and other alterations have not 

 been sufficiently radical, however, to call for new drawings 

 at this time. 



The magnet used with this new apparatus was one which 

 was made in the laboratory. The pole-pieces are 7*2 cm. 

 in diameter, and in these experiments were 3*8 cm. apart. 



Experimental Results. 



The experimental results which follow are classified 

 according to the positive elements of the salts. Curves are 

 included in a few cases ; for the most part, however, only 

 numerical data appear. 



Barium. 



The salts of barium which have been examined are the 

 sulphate, the phosphate, the fluoride, and the chloride. 



The barium sulphate used was prepared from Kahlbaum's 

 barium chloride and Kahlbaum's ammonium sulphate. Small 

 droplets of distilled water holding particles of the sulphate 

 in suspension were vaporized on the upper surface of the 

 strip, which had been previously heated to rid it of its own 

 positive emission. After determining d, the apparatus was 

 assembled and pumped down by means of a Gaede pump. 

 While the evacuation was being effected the strip was heated 

 gently to drive off what moisture might remain in the salt. 

 When the pressure was reduced to less than fx 10~ 4 mm. 

 of mercury by the McLeod gauge, a potential of 320 volts 

 was established between the plates A A and BB, A A being- 

 positive to BB. On increasing the heating current gradually, 

 a thermionic current was first noted before the strip became 

 incandescent. This current was very transient, however, 

 decreasing below the limit of possible measurement in a very 

 few minutes. Similar momentary emissions were noted tor 



