80 
IOWA ACADEMY OF SCIENCE Voi,. XXVIII, 1921 
point is ill contact with the pyrometer. The motor cannot be at- 
tached to the pyrometer on account of the vibration which it is 
almost certain to produce. Some additional sectored-discs are 
shown in the figure. By using combinations of two discs, sectors 
having various transmissions can be formed. The writer has used 
sectors having openings varying from 1° to 230°. 
A/" is a Siemens & Halske Precision volt-ammeter, Type 7K. It 
is equipped with an external adjustable shunt having the follow- 
ing steps: 0.424, 0.322, 0.254, 0.203 and 0.150 ohms. The i'lternal 
resistance of the meter is 10 ohms and it requires 0.0045 ampere 
to produce a deflection of 150 scale-divisions. This ammeter with 
shunt was calibrated at 80, 100, 120 and 140 divisions for each 
shunt-step by using a. standard resistance with a Leeds & North- 
rup potentiometer. Current values when the pointer is deflected 
100 divisions are respectively 0.07319, 0.09623, 0.12033, 0.15044 
and 0.20115 ampere. 
The pyrometer lamp has a filament about 0.033 mm, in diameter 
and it requires a current of 0.2350 ampere to match in brightness 
a black-body at 1828°K. This current causes a deflection of 116.7 
divisions when using the fifth shunt ; it is the largest current that 
should be sent through this lamp, since lamps with tungsten fila- 
ments will not remain constant for any reasonable length of time 
if used to match bodies whose brightness-temperatures are greater 
than 1828°K. The lowest temperature that can conveniently be 
measured when using the colored glasses is about 1080°K but the 
curve can be extended down to 975 °K by removing the glasses. 
The current required through the pyrometer lamp when a temper- 
ature of 975° is being measured is 0.0818 ampere. The values of 
the pyrometer-lamp currents given in this paragraph refer to the 
particular settings of the lens, 'lamp and telescope shown in figure 
7. The aperture of A was U. S. 4 and the discs having apertures 
of 13 mm. and 15 mm. were used at 1828 and 975° respectively. 
In series with pyrometer lamp is the storage battery O, the 
ammeter N and the two rheostats P. The rheostats are con- 
nected in parallel ; one has a resistance of 40 ohms and the other 
a resistance of 107 ohms. A two-volt battery is sufficient when 
measuring the lower temperatures but a four-volt one is necessary 
for most of the range. 
With the set-up shown in figure 7 the background is magnified 
about seven diameters. Very often such large magnification is not 
desirable. With the instrument here described, much smaller mag- 
nifications can be used if the lamp and lens are moved closer to- 
