NATURE 
[Juty 18, 1912 
carbon (Fig. 2), or spiral if made of Acheson graphite | a furnace was talken out after it had been run for 
(Fig. 3). In the latter case they are provided with an 
GRAPHITE SPIRAL 
LINER TUBE 
| 
| 
| 
| 
r 
a} 
L 
Fic. 3.—Carbon-tube furnace with graphite spiral Theater. 
internal liner tube of carbon. There is no special 
difficulty in cutting the spirals from the solid; 
graphite, unlike amorphous carbon, is an extremely 
tractable substance to machine. 
We have used these carbon resistance furnaces a 
great deal at the National Physical Laboratory, and 
Mr. Greenwood, at Manchester, carried out his 
experiments on boiling metals by the aid of such a 
furnace. The boiling of a metal forms a not impos- 
sible lecture experiment, and a projected image of 
the surface of boiling tin (shown) displays all the 
usual phenomena of ebullition. The heating up of 
carbon is somewhat strikingly shown by passing a 
heavy current through a thin broad carbon strip pro- 
vided with water-cooled terminals (experiment shown). 
The lines of flow from one terminal to the other are 
well illustrated at one stage of the heating. 
Among other methods of electric heating are the 
induction furnace, which is of great value in refining 
crude materials, and the flame spark, in which it is 
possible to volatilise so refractory a substance as an 
incandescent gas mantle. 
Some time ago I endeavoured at the National 
Physical Laboratory to make a furnace for very high 
temperatures without employing carbon.’ The intro- 
duction of the Nernst lamp was suggestive. It was 
found that a great number of substances could be 
made to act like a Nernst filament, e.g. a piece of 
the stem of a churchwarden pipe, if sufficiently 
strongly heated, can be made to conduct electricity 
well enough to become ineandescent. Carborundum 
crystal behaves similarly, and requires no initial heat- 
ing (experiment shown); in this case the temperature 
can be raised high enough to volatilise off the silicon, 
which burns, forming a cloud of silica. A cascade 
furnace was constructed on these lines: a tube made 
up of zirconia and a little yttria was raised by means 
of an insulated nickel winding to 500° or 600°, at 
which temperature the tube conducts sufficiently well 
to enable a heating current to be passed through it. 
There is no difficulty in melting platinum, for ex- 
ample, in such a furnace using a quite small heating 
current (about 2 amps.). A zirconia tube from such 
5 Pioe. Roy. Soc., vol. 76.A, p. 235, 1905. 
NO. 2229, voL. 89] 
a 
LAMP BLACK 
BRICK SUPPORT 
COPPER TUBE 
six months or so; it was then found to be quite trans- 
lucent. The possibility of 
constructing in such a way 
refractory gas-tight mate- 
rials at once suggested itself, 
and we proceeded to manu- 
facture ‘“‘pottery’’ at high 
temperatures. Great difficul- 
ties have been encountered 
in the experiments. Whereas, 
for example, the potter in 
baking his wares at tem- 
peratures up to 1300° C. 
looks for a shrinkage of 5 
per cent. or so, we were 
confronted with a shrinkage 
of 37 per cent. with tubes 
baked at temperatures up to 
| about 1800° C. For the pur- 
| poses of the fritting we 
employed carbon-tube — fur- 
naces of one of the types 
; mentioned above. Now it 
WATER TUBES —‘[ a sometimes happened that 
CABLES ‘ha the outer surface of the 
WEIGHTS zirconia tubes, instead of 
having the white and hard 
appearance of the rest, was 
found to be carburised and 
crumbly after baking. The action was _ not 
merely superficial, but extended to an appreciable 
AMPERES 
0 VOLTS 2 4 6 8 
1G. 4.—Relation between ionisation current and applied potentials for 
1 cm. gap between the electrodes. 
5) 
depth. On the other hand, the inner surface of the 
tube, though freely open to the furnace atmosphere, 
