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May, 1925
Briguet spiral to one of the hair springs of the movable
coil. The spiral is made of two dissimilar metals
and tends to coil or uncoil with changes in temperature
due to their different expansion. If the temperature
of the instrument rises, the spiral will tend to uncoil,
and has the same effect as moving the zero adjustment
of the instrument, causing the pointer to move
up the scale an amount equivalent to the rise in cold
ZERO ADJUSTING
SCREW
CONTROLLED BY
BRIGUET SPIRAL
Forging - Stamping - Heat Treating
BRIGUET SPIRAL
FOR AUTOMATIC
COLD JUNCTION
COMPENSATION
FIG. 94—Phantom view of millivoltmeter, showing
internal construction.
junction temperature. A second spiral is attached to
the scale of the instrument, and carries an index to
which the zero reading of the instrument must be adjusted
when the pyrometer is on open circuit.
Another method consists in connecting a coil ol
wire, whose resistance changes considerably with temperature,
in the circuit in such a way that it compensates
electrically for changes in the cold junction temperature.
This method will be described under Potentiometers.
Millivoltmeters have the advantage that they are
direct reading, that is, indicate temperature by the motion
of a pointer on a scale, are comparatively simple
in mechanical and electrical design, and may be somewhat
lower in first cost than potentiometer type to
be described later.
They should, however, be frequently checked. They
are not entirely independent of changes in line resistance,
poor connections in the line may greatly effect
their readings, their accuracy is dependent upon the
constancy of the permanent magnet and of the controlling
springs, both of which are subject to variation
with time and temperature. Any dulling or clogging
of pivot and jewel bearings will affect their reliability,
and may come about through jarring of the instrument
or by the access of dust or moisture.
PART 5. — POTENTIOMETERS
The potential in the slide wire will vary with the
current flowing through it from the cell Ba and a
We have seen that the millivolemeter type of tem­ means for standardizing this is provided. S. C, Fig.
perature indicator really measures the current flowing
in the circuit, and that its readings will not truly represent
the e.m.f. of the thermocouple unless the resistance
of the entire circuit has a definite and constant
value. This difficulty is overcome, and certain other
96-b is a Weston type Standard Cell whose voltage is
constant. It is connected to the potentiometer circuit
at two point D and F by closing the contact
shown between S. C. and Galv., whenever the potentiometer
current is to be standardized. The galva-
169
advantages gained in the potentiometer type of instrument.
The potentiometer measures an unknown e.m.f. by
opposing it with a known e.m.f. which is varied
until equal to the unknown.
The description of this instrument, given by the
manufacturer, the Leeds & Xorthrttp Company, is so
clear and concise that it is quoted below.
"The potentiometer provides, first a means for securing
a known variable potential, and, second, suitable
electrical connections for opposing that potential
tn the unknown electromotive force of the thermocouple.
The two are connected with opposite polarity,
so that they oppose one another. So long as one
is stronger than the other a current will flow through
the thermocouple; when the two are equal no current
will flow.
Fig. 96-a shows the wiring of the potentiometer in
its simplest form. The thermocouple is at H, with its
polarity as shown by the symbols -j . It is connected
with the main circuit of the potentiometer at the
fixed point D and the movable point G.
A current from the dry cell Ba is constantly flowing
through the main, or so-called potentiometer circuit,
ABCDGEF The section DGE of this circuit is a
slide wire, uniform in resistance throughout its length.
The temperature scale is fixed to this slide wire. The
current from the cell Ba as it flows through DGE sets
up a difference in potential between D and E. There
will also be a difference in potential between D and
all other points on the slide wire. The polarity of this
is in opposition to the polarity of the electromotive
force of the thermocouple which connects into the potentiometer
at D and at G. By moving G along the
slide wire a point is found where the potential between
D and G in the slide wire is just equal to the electromotive
force generated by the thermocouple- A galvanometer
in the thermocouple circuit indicates when
the balance point is reached, since at this point the
galvanometer needle will show no deflection.
/
Hot Junction
Compensating Leads
r
Millivoltmeter
Binding Posts N Sprinq
PIG 95—Electrical circuit of millivoltmeter and couple.