Ionization from Certain Hot Salts, 227 



The formula connecting these quantities is : 



e 



m 



9 V; 



* 



2 Eft 



(1) 



where V = potential, H = strength of field, d = distance 

 between strip and plate which receives ions, and x = half 

 the deflexion caused by reversal of field. 



All these quantities were measured as described in Prof. 

 Richardson's article on " Positive Ions from Hot Metals" f. 



The platinum strip was cut with narrow and short con- 

 strictions, one on each side of the centre. When a current 

 was passed through, these constrictions would become hotter 

 than the rest of the strip, and thus would tend to hold in 

 place a salt lying between them. 



The accuracy of the results of the experiment was found 

 to depend upon the accuracy with which the distance d could 

 be measured and kept constant. To accomplish this end two 

 methods of mounting the strip were used : — 



(a) A very short strip was made ; its ends were soldered 

 to two heavy brass strips connected to the lead wires for the 

 heating circuit. It was thought that the shortness of the 

 strip would diminish the bending due to changes in tempera- 

 ture and hence length. This method was found to be not 

 altogether satisfactory. 



(h) A longer strip was used, fastened at one end rigidly 

 to the end of one lead wire, and at the other to a phosphor- 

 bronze spring acting as the other lead wire. This spring 

 was under tension sufficient to take up any increase in 

 length and to hold the strip level. When carefully adjusted 

 this mounting was very successful. 



The making and successful mounting of a perfect strip is 

 rather a delicate operation. This, together with the inevit- 

 able subsequent variations in the distance d and many other 

 difficulties, made an exact experiment a tedious process. 



The temperature-resistance curve of a test Pt strip in air 

 was drawn by taking the following as fiducial points : — 



Room temperature. 

 Boiling water. 

 Melting point of NaN0 3 . 

 ,, ,, ,, Borax. 



,, ,, ,, JNa 2 b(J 4 . 



* Richardson, Phil. Mao-., Nov. 1908, p. 740. 

 t Roy. Soc. Proc. A. vol. lxxxix. p. 507 (1914). 



Q2 



