
JuLY 22, 1915| 
NATURE 
577 


by keeping F fixed, the whole of the scale may be 
utilised for reading from 500° upwards. A more 
open scale, with correspondingly closer readings, may 
thus be obtained; and by suitable adjustment of F 
the range covered may be varied as required. The 
same firm has also introduced a mechanical device 
for achieving the same object. In this a suspended- 
coil indicator is used, and by turning a torsion head 
a twist.is imparted to the suspension, so that the 
pointer does not move over the scale until the tem- 
perature of the junction has reached an assigned 
figure. In the multi-range instrument made by R. W. 
Paul, the indicator takes the place of the galvano- 
meter in a Wheatstone bridge circuit, the pyrometer 
being in series with the indicator. By throwing the 
bridge slightly out of balance, a current may be made 
to pass through the indicator in an opposite direction 
to that produced by the heated junction; and only 
when this opposing current is overcome will the 
pointer begin to move over the scale. Resistances are 
‘provided which, when inserted in the arms of the 
bridge, disturb the balance so as to hold up the in- 
say 

dicator until an assigned temperature 600°—is 

Fic. 2. —Pyrometer for superheated steam. 
reached, when the whole scale becomes available for 
the selected range. By using two mangani:: and two 
copper resistances in the arms of the bridge, auto- 
matic correction is made for changes in the tem- 
perature of the cold junction. Thus, if the bridge 
were balanced at 20°, a current would flow through 
the indicator at any other temperature, as copper 
changes its resistance on heating or cooling, whilst 
manganin does not. The resistances are so chosen 
that the movement of the pointer caused by this cur- 
rent represents the increase or decrease in the tem- 
perature of the cold junction, and thus any errors due 
to this cause are eliminated. 2 
The regulation of the temperature of superheated 
steam on locomotives has given rise to the problem 
of producing an indicator practically unaffected by 
vibrations. This has been solved in various ways 
by different makers, and satisfactory instruments are 
now procurable for this purpose. Fig. 2 shows a 
pyrometer of this type made by Messrs. Siemens. 
Altogether, great progress has been made in this 
branch of pyrometry, particularly in the direction of 
greater accuracy. 
Senet 2386; VOL. Osi 

Resistance Pyrometers. : 
No special change is to be recorded in connection 
with this class of instrument. The recent worl: of 
Sir William Crookes has shown that platinum is dis- 
tinctly volatile above 1o00° C., and this explains 
satisfactorily why resistance pyrometers were found 
to change their calibration when used above this 
temperature. For work at low or moderate tem- 
peratures the resistance pyrometer is much used on 
account of its superior accuracy, which, however, 
is only operative when the temperature to be measured 
is subject to precise control. It is now customary to 
employ thermo-electric pyrometers for the general run 
of metallurgical work, and to use a resistance pyro- 
meter for very exact work, and for a workshop 
standard within its upper limit—-iooo° C. 
The researches of Northrup on the resistance of 
molten metals (Journal of the Franklin Institute, 
~~ 
f 
‘ 
Ks 

Fic. 3.—Foster’s radiation pyrom: ter for molten metals. 
January and March, 1914) suggest a possible exten- 
sion of the range of resistance pyrometers by the 
use of a liquid element. Melted copper, for example, 
shows a uniform increase in resistance up to 1400° C., 
and this fact might be utilised in measuring tempera- 
tures if a suitable appliance were forthcoming. The 
decrease in the resistance of pyro-conductors with 
rise in temperature may possibly be utilised for resist- 
ance pyrometers. Alundum, for example, has a resist- 
ance of 6100 ohms per cm. cube at 1100° C., which 
falls to 190 ohms at 1600° C.—-an average decrease 
of nearly 12 ohms per degree, which could easily 
be detected on a coarse instrument. 
Radiation Pyrometers. 
A distinct improvement in these instruments has 
been the introduction of pivoted indicators in place 
of the suspended-coil type formerly in use. This has 
