172 . Dr. C. Shear d on the Positive 



If the " induced " ionization be denoted by X, then 



X — n — — —Jr~P — Ir\ fi~ kt — A />-** 



x ~ n dt~ Tt~ k ^- kAie " A " ' 



so that logX=C— Jet. 



In both sample sets o£ readings given above, if we plot 

 the logarithm of the difference between the actual current 

 and the slow-decay or " steady " current, a straight line is 

 obtained. Later in this paper, it will be shown that at least 

 two substances capable of producing ions under heating 

 must be assumed, and that the simple logarithmic law given 

 above is not generally applicable. 



Complications arose, however, when fresh gas (air) was 

 admitted to the tube and allowed to remain there several 

 hours, the wire being cold during the interim. Upon re- 

 pumping to a low pressure, it was found that the current- 

 E.M.F. curves were either slightly convex to the voltage 

 axis or almost linear. With subsequent heating under 

 potential the usual saturation-current conditions obtained. 

 The positive thermionic current from the wire was found 

 to be initially about 15 to 20 times as great as the value 

 previous to the admission of air. This increased current 

 fell off rapidly with time and reached the slow-decay stage 

 in about 15 minutes. A preliminary account of some investi- 

 gations along this line, made by Richardson and Sheard, is 

 to be found in the Physical Review, vol. xxxiv. p. 392 (1912). 

 Some experiments made recently by the writer of this paper 

 indicate that these effects are due in large part to small 

 quantities of water vapour. It was decided, therefore, to 

 carry out the present investigations in dry gas at atmospheric 

 pressure. This ensured the reproducibility of conditions 

 when the apparatus had been opened, the wire treated and 

 re-inserted, and the thermionic currents again investigated. 

 Also any effects due to the contamination of the wire with 

 mercury or phosphorus vapour were eliminated. 



II. Apparatus, 



The main chamber used in these experiments consisted of 

 a cylinder of brass, 11*5 cm. long and 6 cm. diameter, closed 

 at the two ends with brass plates. Through the lower plate 

 two insulated heavy leads were passed, and across these was 

 fastened the platinum wire 0*01 cm. in diameter and about 

 3 cm. long. The receiving electrode was a hollow brass 

 cylinder, 4x1 cm., closed at both ends and attached to a micro- 

 meter-screw passing through a threaded bushing of brass 



