Section III., 1911. [53] Trans. R. S. C. 
The Relation Between the Current from Hot Platinum in Air at At- 
mospheric Pressure and the Electric Force. 
By, PROF H. Ay) Witson, FIRS.) Fane SIC 
(Read May 17, 1911.) 
In a paper * published in 1901, the writer determined the relation 
between the current and the potential difference between two concentric 
cylinders of hot platinum in air at atmospheric pressure. The object 
of the present paper is to show that the observed relation can be ex- 
plained theoretically, and that the velocity of the positive ions which 
are emitted by the hot platinum, can be deduced from the observations. 
The current between the hot cylinders is carried entirely by positive 
ions emitted by the one which is positively charged. Negative ions 
are not emitted in appreciable quantities unless the temperature is 
above about 1,200° C. 
In the case of two concentric cylinders, the electric force F is given 
by the equation 
d 
— (Fr) = 4rpr 
dr 
where p is the density of the electrification in the gas and r the distance 
from the axis. The current C per em. of length along the axis is equal 
to 2 rt Fpk, where k is the velocity of the ions due to unit field. Hence 
d ACr 
—~(Fr)? 3 Uh yess 
dr k 
so that 
2C 
Er) — (Er)? = Nel gage i) epee ae) 
where F, is the field at radius r, and F, that at r.. 
This equation shows that the electric field will not be appreciably 
changed by the ions so long as its right hand side is small compared with 
Fr. | 
* On the Electrical Conductivity of Air and Salt Vapours, Phil. Trans. A., VI. 
197, p. 415, 1901. 
