ength of the
element to the temperature to be measured, as the electromotive force
depends only upon the temperature of the juncture at the closed end of
the protecting tube and that of the cold end of the element. The
galvanometer can be located at any convenient point, since the length of
the wires leading to it simply alter the resistance of the circuit, for
which allowance may be made.

The advantages of the thermo-electric pyrometer are accuracy over a wide
range of temperatures, continuity of readings, and the ease with which
observations can be taken. Its disadvantages are high first cost and, in
some cases, extreme delicacy.

Melting Points of Metals--The approximate temperature of a furnace or
flue may be determined, if so desired, by introducing certain metals of
which the melting points are known. The more common metals form a series
in which the respective melting points differ by 100 to 200 degrees
Fahrenheit, and by using these in order, the temperature can be fixed
between the melting points of some two of them. This method lacks
accuracy, but it suffices for determinations where approximate readings
are satisfactory.

The approximate melting points of certain metals that may be used for
determinations of this nature are given in Table 8.

Radiation Pyrometers--These are similar to thermo-electric pyrometers in
that a thermo-couple is employed. The heat rays given out by the hot
body fall on a concave mirror and are brought to a focus at a point at
which is placed the junction of a thermo-couple. The temperature
readings are obtained from an indicator similar to that used with
thermo-electric pyrometers.

Optical Pyrometers--Of the optical pyrometers the Wanner is perhaps the
most reliable. The principle on which this instrument is constructed is
that of comparing the quantity of light emanating from the heated body
with a constant source of light, in this case a two-volt osmium lamp.
The lamp is placed at one end of an optical tube, while at the other an
eyepiece is provided and a scale. A battery of cells furnishes the
current for the lamp. On looking through the pyrometer, a circle of red
light appears, divided into distinct halves of different intensities.
Adjustment may be made so that the two halves appear alike and a reading
is then taken from the scale. The temperatures are obtained from a table
of temperatures corresponding to scale readings. For standardizing the
osmium lamp, an amylacetat