DISCHARGE OF ELECTRICITY FROM GLOWING CARBON. 201 
THE DISCHARGE or ELECTRICITY From GLOWING 
CARBON. 
By J. A. PoLuock, Professor of Physics in the University 
of Sydney, and A. B. B. RANCLAUD, B. se. 
[Read before the Royal Society of N. 8. Wales, September 2, 1908. ] 
1. Introduction.—The experiments, of which a description 
is here given, were undertaken in connection with an 
investigation of the phenomena associated with the relight- 
ing of the carbon arc. For the object in view an arrange- 
ment was required in which carbon rods should be situated 
somewhat as they are in an arc lamp, and in which the 
temperature of one of the rods could be readily controlled. 
The plan adopted was as follows:—a cylinder of carbon, 4°5 
centimetres long and 0°5 centimetres in diameter, was 
electrically heated, and a circuit arranged to include an 
air gap, of a few millimetres, between the middle point of 
the heated cylinder and the end of a comparatively cool 
carbon rod. The currents in the circuit have been measured 
for different temperatures of the carbon cylinder, and for 
various voltages across the air gap up to the point at which 
an arc forms, with a view to finding the conditions under 
which the change from the non-luminous to the luminous 
discharge occurs in the case of hot carbon in air at natural 
pressure. The conditions for the similar change in con- 
nection with a hot lime cathode in air at low pressure have 
been investigated by Professor J. J. Thomson.’ 
2. Experimental detail._The apparatus is shown in plan 
and elevation in figure 1. OC, the hot carbon cylinder, is 
<The Conduction of Electricity through Gases,” 2nd edit., p. 477, 
1906 ; see also Horton, Phil. Trans. A., 207, p. 149, 1907. 
