Jury 18, 1912] 
NATURE 
525 
Tae II. 
Temperatures obtainable in :— | 
Deg. C. 
Bunsen burner flame - 1100-1350 
Méker burner flame . 1450-1500 
Petrol blow-lamp flame ... 1500-1600 
Oxy-hydrogen flame -. about 2000 
Oxy-acetylene flame... a Panes 24CO 
Electric arc o¥ Fc cas mais) 3500 
Electric arc (under pressure) ... Beet ssi 3000 
Sun ~e Ae se = Pens 5500 
Some of the methods for measuring temperature 
with their limitations are briefly recapitulated in 
Table III. I have only time to refer to one or two 
points. We have recently had the opportunity at the 
National Physical Laboratory of subjecting a number 
of mercury in silica thermometers to a critical 
examination. These thermometers, which are made 
in England, possess in a high degree the qualities of 
constancy, large range, and such complete freedom 
from temporary zero change, that I feel safe in pro- 
phesying they will inevitably replace the present inter- 
national standards, which are made of verre dur. 
SPACE FILLEO WITH 
LAMP BLACK 
In regard to high temperatures, most of us rely 
to some extent on colour in estimating temperature. 
Table IV. gives a very fair notion of the temperature 
we may reasonably associate with the colour of a 
fire or muffle furnace (experiment shown). The 
intensity of the light varies according to well-known 
laws which have been studied up to sun’s tempera- 
ture. If we know the law of variation we can 
measure the temperature by the use of some kind of 
photometer—which is what all optical pyrometers are. 
Taste I1V.—Temperature and Colour of a Fire. 
Colour. Cent. Fahr. 
‘*Grey,” lowest discernible temp. About 450 | About 850 
Very dull red . 500 Pf 950° 
Dull red ... 700. Ne -155 8) SORE 
Cherry red goo s58 | LO5Or 
Orange 1100 2000° 
White + 1300 +> 2400" 
Dazzling white Above 1500° | Above 2750° 
For obtaining really high 
temperatures electric  fur- 
naces are our only resort. 
Small gas furnaces can 
reach 1600° with difficulty ; 
large industrial furnaces 
attain 1800° C. in some in- 
stances. 
Mr. Cook, of Manchester, 
has kindly lent me for this 
occasion a number of electric 
furnaces. These are con- 
structed by winding tubes of 
fire-clay or alumina with 
nichrome or platinum wire 
or strip: the external lag- 
ging is of  kieselguhr. 
Steady temperatures up to 
about 1000° and 1200° C. re- 
spectively can readily be got 
with power from a commer- 
: cial circuit of 100 or 200 
' volts. With thicker wires 
, and current at lower voltage 
these upper limits can be 
H appreciably extended. 
' For higher 
$ 
DS temperatures 
' oneaneas we have to make use of 
(argent = -------- Alea 1 ie ade eaices eee ee 4: carbon or graphite, and elec- 
as rg tric heating was first 
Fic. 2.—Small model straight carbon-tube furnace. applied by such means in 
Tasie III.—Some Indication of the Present Range of 
Temperature-measuring Instruments. 
Range in Degrees C. 
Method. 
Practical. Extreme. 
Expansion thermometers— | 
Gas thermometer Up to 12co - 272° to1550° 
Mercury in glass 30053500; | —-44°,, 575. | 
Mercury in silica Ms 30° 55) Ca; |’ = 44°,,. 7oo" 
Electrical thermometers— 
Platinum resistance ... —100°,, 1100 | —250°,, 140U" 
Thermocouples : 
— platinum alloys 300° ,, 1400" Up ,, 1750 
— base metals $ —100° ,, 1100" | —250° ,, 1200 
Total radiation pyrometer 500 ,, 1400 | No upper limit 
Optical pyrometers... 600° ,, 3500 | No upper limit | 
NO. 2229, VOL. 89] 
the form of the arc furnace. 
| Such a furnace has many inconveniences—the heat- 
ing is intensely local, and there may, for example, be 
a gradient of 2000° C. in a single inch. There is 
practically no temperature control, and there is every 
possibility of the final product becoming largely con- 
taminated with carbon. Most of the early isolated 
so-called elements have since proved to be largely 
carbides. 
Resistance heating is usually much more con- 
venient, and this is the principle of carbon-tube 
furnaces, some essential features of which were 
employed by Prof. Dewar many years ago. They will 
stand rough use, and are much more controllable 
than the arc furnace. It is as easy to control a 
temperature of 2500° C. as one of a red heat. 
Such furnaces usually have their end-terminals 
water-cooled, and are surrounded by lagging of lamp- 
black or charcoal 
The furnace tubes are either straight if made of 
