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April, 1925 Ibrging- Stamping - Heat Tieating 129
Cold Junction.
Suppose the two wires of a thermocouple are joined
together at the end away from the furnace, or "cold end",
as well as the hot junction, as in Fig. 78. An emf. will
be generated at the cold junction, which will depend upon
the temperature there. This is true because any pair of
dissimilar metals, at any temperature (except perhaps absolute
zero) always generate an emf. when in contact.
W V W v *
FIG. 80—Optical pyrometer, diagram.
In any junction, the two wires are like the poles of an
electric battery, one being plus and the other minus, depending
on the metals used. The current tends to flow
away from the junction in the plus or positive wire, and
toward it in the negative or minus wire. It is evident in
Fig. 78 that the emf. of the cold junction will oppose that
of the hot junction, as it tends to send current in the opposite
direction.
If E is the e.m.f. of the hot junction, and e the e.m.f.
of the cold junction, the effective e.m.f. in the circuit
will be E — e = E'. The current in the circuit will be
R R
R = Total resistance of cricuit.
In order to measure the current or the e.m.f. in the
circuit, it is necessary to connect the wires of the couple
to a measuring instrument. The connectors of the instrument
may be of brass and the internal wiring of copper,
manganin or other metal. Every joint between dissimilar
metals in the circuit sets up an e.m.f. Some will
act in the same direction as the e.m.f. of the hot junction
and others will act in the opposite direction. In
any thermocouple circuit, if there are several junctions
all at the same temperature, the algebraic sum* of their
electromotive forces is zero. Since the measuring inefru •
ment is (or should be) at uniform temperature throughout,
all junctions within it may be disregarded. The
circuit will, therefore, behave as though the point at
which the wires of the thermocouples join the instrument
were the cold junction, see Fig. 79. The cold junction
e.m.f. will equal that which would occur were the couple
wires joined to each other where they enter instrument,
or e. In other words, the cold junction temperature is
•This means that those acting in the same direction as the
hot junction are added, and those in the opposite direction are
subtracted.
the temperature of the point where the thermocouple
wires end.
It is evident that, in order to determine the temperature
of the hot junction, allowance must be made for
the temperature of the cold junction. Methods for doing
this will be described further on.
Optical and Radiation Pyrometers.
Long before the invention of pyrometers, workers
with steel estimated the temperature of the hot metal
by eye, by judging its "color" Sometimes they achieved
remarkably accurate results. When they did not, the
consequences of their error were generally blamed on
"bad steel" or an evil spirit.
When an object is heated to a temperature above that
of its surroundings, it gives off energy in the form of
heat or light. This energy is radiated from the object
as waves. If the temperature is high enough, some of the
waves become visible to the eye as light, and the object
glows red, orange, straw color, etc. It becomes "incandescent".
Even before the piece is hot enough to glow
we can feel the radiant heat by holding our hand near it.
The energy given off by a body in this way, either
as heat or light, or both, bears a certain relation to its
(absolute) temperature. This principle is used in the
measurement of high temperatures. It is the old method
of eye or feeling developed to a scietific basis, by eliminating
the personal element, and setting up accurate
and reproducible standards.
Pyrometers which utilize the radiation from a bodyare
divided into two classes, known as "optical" and "radiation".
The first measures the light given off from the
hot body, sometimes by separating out a single color,
such as red, from all the light emitted, and comparing
the intensity of this one color with the intensity of the
FIG. 81—Using optical pyrometer.
same color given off from a standard source of light.
The comparison is made by eye, as the eye is very sensitive
to a difference between the brightness of two small
bodies which are close together. The second class measures
all the radiation, including heat and light, which
falls upon a receiving surface in the instrument. Usually
this radiant energy is focused upon the hot junction
of a small thermocouple, and the consequent rise in tern-