DISCHARGE OF ELECTRICITY FROM GLOWING CARBON. 203 
ance being made, of course, for the potential gradient in 
the hot rod due to the heating current. A fine wire fuse 
was used to protect the galvanometer from excessive 
currents. 
When the cold carbon is close to the hot one its tempera- 
ture rises, and the current varies with time. To secure 
uniformity, measures have been taken in the following way: 
with the cool carbon removed, the hot cylinder was brought 
to its full temperature; then by tilting the holder E, 
figure 1, the cool carbon was put quickly into position at 
the required distance from the cylinder; in 6to10 seconds 
after this the galvanometer deflection became fairly steady, 
and all observations have been taken within these limits 
of time. 
The heated carbon cylinder was a piece of the carbon 
rods supplied by Messrs. Siemens Brothers for use with 
their Lilliput arc lamps, while the cooler carbon was a 
portion of Conradty arc lamp carbon, turned down at the 
end as shown in the figure. 
The temperature of the heated carbon was found with 
the aid of a Holborn-Kurlbaum optical pyrometer, calibrated 
at the Reichsanstalt. The temperatures are given as 
observed, they are therefore in all cases the equivalent 
black body values. 
