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Xill, TIk' Electrical ( \mflHCtivihj Trnparfed to a Vacuum luj Hot (ouductors. 
By O. W. Kichaedson, B.A., B.Sc., Felloiv of Trinity Colleye, Cambridge. 
Communicated by Profe ssor J. J. Thomsox, F.R.S. 
deceived Felmiaiy 28,—Eead March 2C, 1903. 
TxTEODUC’TIOX. 
The ex23erimental pai't of the present paper is an investigation of the electrical 
conductivity of the space siuToiinding hot surfaces of platinum, carbon, and sodium 
at low pressures. A preliminary account of some of the experiments on platinum 
was read before the Cambridge Philosophical Society on November 25th, 1901.'" 
The conductivity produced by hot metals has been the subject of a great numbei' 
of researches liy different authors. The phenomena are, however, very complicated ; 
for the quantity and sign of the ionisaticni is found to vary in the most remarkable 
manner with tlie nature, temperature, and previous history of the metal, with the 
nature and pressure of the surrounding gas, and with small changes in the state of 
the metal surface. The present investigation was undertaken with the idea that in 
the negative ionisation at high temperatures the conductivity produced by metals 
took its simplest form. This idea is supported by the observation of Professor 
McClelland, I that the negative current is to a great extent independent of the 
nature of the gas, and is independent of its pressure over a luiige from '04 to 
’004 millini. 
1 he chief problem whicli is here attacked experimentally is the way in which the 
saturation current from the hot metal surface to a neip’hbourino; electrode varies with 
the temperature of the metal The value of the saturation current corresponds to 
the total number of ions which are produced by the surface per second. Incidentally 
it was found necessary to examine, in addition t(3 the aliove, the relation between the 
current and the electromotive force for the conductivity produced by tlie tliree above- 
mentioned conductors at various pressures. 
The theory, by whicli it is proposed to explain the plienomena, is based on the 
* ‘Proc. Gamin’. Pliil. Soc.,’ vol. 11, ]). 28G. 
t ‘Proe. Cambr. Phil. ,Soc.,’ vol. 10, p. 241, and vol. 11, p. 296. 
VOL. cci.— A 343. 
20.7.03 
