Download thermojunctions

JUNE 1938
ELECTRICAL PHENOMENA IN THE POSITIVE COLUMN AT LOW PRESSURE
is more advantageous to bring the atom from the
4.86 volt state, to which it has fallen after emission
of the blue line, immediately back again to the
7.69 volt state, than to wait until the atom
has returned to the zero state and emitted the
ultraviolet line 2537 A. In the first case only
7.69 - 4.86 = 2.83 volts are needed and this energy
is radiated entirely in the visible region; in the
second case, however, 7.69 volts are needed ofwhich
only 2.83 volts are transformed into visible light.
These cumulative effects are very much favoured
in the case of mercury by the fact that 4.86 volts
is a "resonance level" and 5.43 volts a so-called
"metastabie level". Since line 2537 is a resonance
line, it is readily absorbed by' the atoms, in the
. zero-state. After about 10-7 sec emission again takes
place, and the energy radiated can then' be absorbed by another atom in the zero state. This
process is repeated very' many times before the
resonance radiation leaves the tube, so that the
concentration of atoms in the 4.86 volt state ID-
165
creases sharply and with it the chance of cumulative
excitation from this level to a higher one. For the
metastable 5.43 volt level also the chance of
cumulative excitation is large. If the atom has been
brought into the 5.43 volt level by collision with
an electron, it cannot easily return directly to the
zero state byemission of radiation, it usually does
so by means of collision with other atoms or electrons. The life of such a metastable atom, about
10-4 sec., is much longer than that of an atom in an
ordinary excited state, so that the chance of cumulative excitation is thereby increased. In the low
pressure mercury discharge used. in the blue
illuminated advertising signs, concentration of the
current therefore is an advantage. In the high
pressure mercury lamps this phenomenon is even
more stimulated by the compression 'of the dis.charge to a narrow zone near the axis, .'
Fig. 11 gives a schematic survey of the various
processes here discussed which occur in the positive
column.
THERMOJUNCTIONS
by
J. W. L. KÖHLER.,
621.317.7.082.62
The construction and action of thermojunctions are discussed in this article. A detailed
accoun~is given of the factors which affect the sensitivity and the characteristic of such
junctions. The choice of the meter movement and the classification of thermojunctions
are explained. Finally a survey is given of Philips thermojunctions.
Introduction
article we shall discuss the last type of instruments.
When an alternating current of very low frequency
is sent through a suspended coil galvanometer which
has its zero point in the middle of the scale, the
meter indicates the current at every moment; the
pointer moves back and forth with the frequency
of the alternating current. If the frequency is raised,
then after a moment of resonance the deflection
becomes smaller and' the pointer finally remains
. practically still due to the fact that the moving
. system of the 'meter cannot follow the high frequency. A .suspended coil galvanometer is therefore
not suitable for measuring alternating currents.
Such currents can only be measured with an
.Instrument whose deflections are always in one,
direction, no matter what the direction of the
current, and whose pointer can therefore adjust
itself to definite average deflection. This condition
is fulfilled by dynamometers, soft-iron meters,
rectifying meters and thermal instruments. In this
t
a
In thermal instruments use is made of the heat
development in a resistance, when a current passes
through it. Etis obvious therefore that the direction
of the current can have no influence on the deflection
of the meter. The heat developed can be used in
various ways:
.
i). The thermal expansion of the resistance filament can be indicated by a pointer in some
way or other .
b) The heat developed can be used to heat a
quantity of gas whose increase in volume is
indicated by a drop of liquid in a capillary.
c) The change in resistance of a filament forming
part of a Wheatstone
bridge may .be
measured.
d) The heating of the filament can be measured
with a thermocouple connected to a direct
current meter.
We shall treat the last method in detail,
166
Vol. 3, No. 6
PHILIPS TECHNICAL REVIEW
Construction and action of thermo-electric ammeters
KIe m en cic was the first to construct a measuring
instrument on this principle. The'instrument consists
of two wires of diffe~ent materials which form a
thermo-electric couple, The two wires are wound
together, knotted together or welded together at
the middle (seejig. 1). The current to be measured
Fig. 1. Kl emenë ië 's thermocouple.
The wire of one metal
is fastened to terminals 1 and 4, that of the other metal to
terminals 2 and 3. The wires are joinedin the middle electrically
in some way. The current to be measured passes through
along the terminals 1 and 2, the galvanometer is connected
between 3 and 4.
is supplied to the terminals 1 and. 2; the wires
bèè~me warm" at the' middle due to the heat
development; this causes a thermal electromotive
force between the terminals 3 and 4. This voltage
can be measured by' connecting a sensitive meter
to the terminals.
There are various objections to this construction
which will appear later. Fig.2 gives a sketch of
Fig. 2. Modern thermocouple.
Between terminals 1 and 2
a filament is stretched, and between 3 and 4 there is a thermocouple whosejunction is fastened to the fila~ent.
electric ammeter. If. a current i is sent through
the filament with the resistance Rg, an amount
of heat equal to i2Rgt will he developed in the
filament. ,The temperature
of the filament will
_'thereby be raised; the heat is lost by conduction
over the terminals,
and by conversion into
electrical energy through the thermocouple and
by radiation. If the amount of heat which is given
up is proportional to the increase in temperature,
the latter will be proportional to the heat developed,
i;e. to i2• The same is true of the temperature
of the junction of the thermocouple. If the thermal
electromotive force is proportional to the temperature difference, it will also be proportional to i2,
and the same is true of the current in the circuit
comprising thermocouple and galvanometer, which
current' is proportional
to the thermal electromotive force. The deflection
ofthe galvanometer is therefore
proportional
to the square
of the current
in the filament.
The ratio between the current through the
galvanometer and the current through the filament
is called the characteristic of the thermocouple.
Here the characteristic ' is quadratic. Therefore,
with a meter having a quadratic scale division,
after calibration of" one point on the scale, the
primary current may be read off directly. A meter
with a linear scale can àlso be used, the reading
is then proportional
to i2,' i..e. to the cnergy
which can be developed by the prim~ry current
in a resistance; this method of measurement has
many practic~l applications.
Mathematical
analysis
We shall give a brief account of the mathemaa modern thermo-electric ammeter from which it
tical analysis
of the action of' the thermo.'may be seen that the principle is the same as that
ammeter. We shall first consider 'the filament
of the original. A filament is stretched between
alone without the thermocouple, The following
1 and 2. Midway between the terminals a thermodifferential. equation holds for the teinperature
couple is fastened -to the filament with its junction
increase in.a straight wire stretched between two
on or close to the filament. The ends of the thermocouple are fastened to the terminals 3 and 4, ' points and heated by a current:
between which the direct current
ammeter is
d21'
au
, Ai2(!
= 0 .
(1)
connected. The whole is often placed in an evacuated
dx2
Aq
Aq2
bulb. When this is done there is no loss of heat
Where:
by convection, and the sensitivity of the instrument
is increased. In certain types the thermocouple is x the coordinate of length of the wire from the
middle in centimetres,
electrically connected with the filament, in others
r
the
tempera~ure difference with respect to the
it is insulated from ,the filament. This construction
surroundings
at the point x in degrees centiis not so simple as the original one, but it has
grade,
certain advantages. One of these is immediately
u. the circumference of the wire in centimetres,
. obvious. 'The thermo-electric
properties
of the
q the area of the cross section of the wire in square
filament material need not be considered, so rhat
centimetres,
the choice becomes muclî greater.
i the electric current in amperes,
We shall now discuss the action' of'.the thermo-
- - - •+-
- I
THERMOJUNCTIONS
JUNE 1938
A the electrical equivalent of heat, 0.2388 calories
per watt second,
e the ,specific resistance of the wire in ohm centimetres,
a the heat radiation in calories per centimetre
second,
A 'the specific conductivity for heat, in cal/cm
sec. degr. C.
240
T't'
, (dor)
and ( -d.) . By heat
dx2
dXl
t
. A~ [( :)
1-
Ai2e
An amount of heat -q
( :)
J
dx .
'
dx is developed in the element by
/
,120
,I
, 80
40
à
;;/
-
<,
/
N.
J
/""
- r-, V
-
"'-
0,4
"'-
~
\
" '\
~'
I{'_
0.5
0,3
G,2
0,1
0
0,1
02
0,3
x
.
= Aq ::
;I"
160
-T~
conduction' therefore
the following amount of energy is supplied to the element:
.
I-
.",
200
Equation (1) can be derived as follows. Consider a length
dx of the wire; at the ends of this element the temperature
gradients are
167
0,4
'~+T
.e66S'
os
x
Fig. 3) a) Temperature of a stretched wire through which
current is flowing,
. b) Temperature of the same wire when a thermocouple is fastened at its middle point.
with the diameter d. The increase of t~mperature
is therefore actually proportional to i2• When K is
small (this is usually the case in practical appliéations
as we, shall see later), we may write:
.
the current ·i. If the element is situated in a vacuum it loses
energy to the surroundings only by radiation; the amount
of this radiation is u a • dx, where it is assumed that the
radiation is proportional to the temperature difference, which
is permissible when the' increases in temperature are small
cosh K = 1 '1(2/2,
(see below for more detailed explanation), In the stationary .
state the energy supplied must equal the energy given off, and (3) becomes:
and therefore
d2•
Ai2e
.
Ai2
A i2 ell
Aq-dx
dx = uo r de,
----=
dx2
q
8 .q Aq
+
+ ~-
8
e,»;
(3a)
from which (1) f1llows.
In this derivation it is therefore assumed that e, A and a
are independent of the temperature; we shall see later what
changes are necessary when this condition is no longer satisfied,
In this expression Rg is the electrical and Rw the
heat resistance of the filament. In (3~) the radiation
constant 6 no longer occurs due to the approximation. This fact will be found later to be very imIf the limiting condition is introduced that the portant.
increase. of temperature at the terminals is> zero
We, shall now pass on to the cas~ where a 'thermo, . (x =
l/2.), then the equation has the following
couple, is fastened to the middle of the filament.
solution:
This can be described in good. approximation by
calculating that a quantity of heat M is removed'
" x ~u -.
cosh
at
the middle of the filament per second and per'
,
A
Ai2e
1":""
q
ï:=--.
(2) degree increase of temperature. The differential
auq
equation then becomes more' complicated; we shall
cosh- l~u 2
Aq
, only give the result for the increase in temperature
tM at the middle of the wire:
(cosh a' is the, hyperbolic cosine of a, i.e. the
\ abreviated notation ofl/2.(ea
e-à); the expression
.1
.hyperbolic tangent which occurs later' stands for
1- --=--=
Ai2e
cosh K
t
ea - e-a·
. (4).
,
). In fig. 3a this increase of temperature is
MtghK
«v«
M tghK
ea
e-a
1
1 ----:==
drawn for' different points of the filament.
2}aAuq
2VaAuq
At the middle the increase in temperature (t) is:
±
+
+
A
t=--
l
i2e 1
auq
where K
~~
l~u
2
-
. auq
V;;
the fact that the heat lost flows to the middle
of the :filament from parts lying near the middle.
For small values of K the following holds:
{ 1-
,
Aq
= 1
+
The increase of temperature
is therefore greater,
1. ~(3)
tanhK
cosh K
. the smaller M. The factor
takes account of
=-Ai2e
1
cosh-
l
+ ,--;;--
for round
wires
...
'YaÀuq
.
PHILIPS
168
(4a)
In this case also the increase in temperature is
proportional to i2• The influence of the heat conduction is less than in (3a). This is clear enough.
In the first case a great increase in temperature
could be attained by a high heat resistance of the
filament; the supply of heat to the middle of the
filament has now become an important factor. In
fig. 3b we gïve an idea of the variation of temperature
along the filament; the middle no longer has the
highest temperature.
.
.
Finally the value of M must be expressed in
terms of the dimensions and constants of the
thermocouple. An analogous differential equation
is valid here, but with different limiting conditions ..
The solution is the following:
_
M 1-
YGlÀlUlql
,
tghKl
Vol. 3, No. 6
TECHNICAL REVIEW
(5)
.
with a given current. The voltage drop caused by the filament,
i Rg = V, is not dependent on the dimènsions of the filament.
lf a definite rise in temperature must be attained with different
currents, the resist~nce of' the filament must be inversely
proportional to the current. The energy necessary for a given
temperature increase, V2/Rg' is inversely proportional to the
resistance, and therefore proportional to the current; thermoammeters for small currents, therefore, possess a greater
energy sensitivity than those for heavy currents.
The foregoing remains true when the second term in the
denominator is small with respect to one; this is the case
when there is only a slight loss of heat through the thermocouple, or when 1/2 Î. q is small. The latter is always the
case with. thermc-ammeters for larger currents (i I/q must
always he constant). If the second term may not be
neglected, which is the case with thermc-ammeters for
low currents, the sensitivity to energy is smaller. A small
heat loss through the thermocouple is therefore favourable
in such a case. Since ,.1,1 and ,.1,2 are usually given (the choice
of material is determined by the thermo-electric properties),
this can be achieved by making ql/Il and q2/12 small. The
.
(!lIl
(! I
resistance of the thermo-couple Rc = -2
-222, however,
.
ql
q2
becomes large as a consequence of. this measure; a limit is'
thereby set to the reduction of 1\-f (see later). The dimensions
of the thermocouple occur only in the form q/I, so that wires
with the same electrical resistance cause the same heat loss.
In the above it is always assumed that the radiation may
be neglected; this assumption is the basis of the approximation
employ~d here,
+
o
where the subscript
are for one of the
The same formula
for the other wire;
4
12
1 indicates that the values
wires of the thermocouple.
with the subscript 2 holds
the length of the wires is
11yGlUl
- and -; Kl = -- ...The loss ofheat through
2
2
. 2
Àl ql
.
the whole thermocouple is:
+M
M = MI
At small values Of Kl
2•
With a given thermoelectric combination the
above considerations are also valid for the thermal
electromotive force E'h. The sensitivity of the
thermo-electro~eter
is of course proportional to
the thermoelectric force per degree. The current
through the galvanometer im is the following,
when- Rm is the resistance of the meter:
(5) becomes:
.
t
M
= 2Àlql
1
·'4
'(Sa)
If we introduce this value of M into (4) the equation
becomes:
A i2 (! 12
tM=
8Àq2
(4b)
--------~~~-------2
1 + -2~-q~
_À14_ql + -À - q_2
~
Z2
m-
E'h
R+
Rm '
e,
,
where Re is the internal resistance
couple.
Connection of the
thermocouple
measuring
of the thermo-
instrument
to the
We may continue with our considerations along
. two lines.
a ) We may begin with a given thermocouple
with resistance Rc, and ask what meter we must
The temperature of the middle of the filament is therefore
use in order to obtain the greatest deflection' with
now expressed 'in the dimensions and constants of the wires
. a definite primary current i. To' answer this we
of the thermo-couple.
must calculate the voltage on the. meter:
Let us first consider the numerator of (4b). Leaving out
the constants of the material we may, write:
t
M
~
'q2
.
_~_"_'i2R2
q2
s'
The dimensions of the filament occur' only in the form I/q;
filaments, therefore, with different dimensions but with the
same resistance undergo the same increase in temperature
Em = E'h
Rm
e; + Re
Em
Eth
Rm/Re
(6)
Rm/Re
1
+
Fig.4 gives a graphic representation of (6). The .
l·equirement· is that the sensitivity to voltage
of the meter . (i.e. the deflection at a definite'
JUNE 1938
THERMOJUNCTIONS
voltage on the. meter) shall be so great that the
meter has the desired deflection with the voltage
calculated from (6). When Rm = Re the thermo-
r
resistance; if the meter has a higher resistance, the
maximum is shifted to the right, if the resistance
is lower, it is shifted to the left.
Em
.É!Il..1,2
1.0
-- -- --- -- -- r--
0,8
/.
V
1--
-
r 1\
--
I--
'I'--"
V
4
-I,
0,2
I!
I1
0,
o
'0
169
l-t--I-a
f.
-Re
2GG!}!!
1
1
1
2
3
4
5
"
Fig. 5. a) Voltage on the meter as à function of the resistance
of the thermocouple at a constant thermo-EMF.
6
Eth
26698
t
Fig. 4. Drop in terminal voltage of the thermocouple, when
current is taken offby the meter. Em/E'h is plotted as a function
of Rm/Re' When Rm = Re ,the most favourable energy transfer.
takes place.
couple gives off the greatest amount of energy to
the meter. A meter with this resistance' therefore
,
deflection
may be the least sensitive to energy (
.
energy
may be smallest); it is usually also the cheapest
meter. A more sensitive meter may, however, also
, he used.
b) We may begin with a given meter, and ask
how the thermocouple must be constructed in
order to give the greatest deflection on the meter.
This problem is not so simple. It is clear that with
'a given thermal electromotive force, and therefore
a given teinperature of the junction, the voltage
on the meter is highest when the resistance of the
thermocouple is zero (see fig. Sa which is the same
as fig.4, but with a different abscissa). We have,
however, seen that the loss, of heat through the
thermocouple becomes greater as the resistance is
lowered, while the temperature increase of the
junction is decreased by greater loss of heat through
the thermocouple. The thermal electro-motive force
is therefore diminished if the resistance of the
thermocouple is reduced (see fig. Sb). The combination of these two effects is shown graphically
in fig. Se, where the voltage on the meter isplotted
as a function of the resistance of the thermocouple
at a constant current through the filament. If we
pass from a high value of the resistance to a low
value, the current through the meter at fust rises
because the total resistance falls, while the thermoelectromotive force changes very little; the current
then falls again' because the thermo-electromotive
force falls arid the total resistance no longer changes
sufficiently. Fig. 5c is valid for a definite meter
,
-:
........
....- ~
'.
b
.
.-Re
2G700
,
Fig. 5. b) Thermo-EMF as a function of the resistance of
the 'thermocouple at a constant current through
the filament.
Em
t
c
--Re
2G701
Fig. 5. c) Voltage on the meter as a function of the resistance
of the thermocouple at a constant current through
the filament.
.
.'
Further discussion ofthe characteristic ofthe thermoammeter
0
In the deviation of the formulae it is assumed
that (j, À and ()do not depend upon the temperature.
The derivation is therefore only valid for small
increases in temperature. If these increases are
not small, the dependence on temperature of these
quantities must be taken into account. In thàt case,
however, the differential equation is insoluble, so
that the, influence 'of this 'dependence must he
estimated in some other way. 'I'his can he done
very roughly. by first giving the constauts in the
solution of the differential equation the values at
room temperature, and then the values at' the
temperature of the middle, of the filament; the
difference between the two results indicates in any
case the direction in which deviations may J;le
expected. We shall' discuss this' for' the "various
quantities which may.he ,depende~t on temperat~re.
0
,
,
Vol. 3, No. 6
PHILIPS TECHNICAL REVIEW
170
With each individual quantity it will he found
that the result of its dependence on temperature
is that the characteristic is no longer truly quadratic.
In favourable cases the deviations due to the different quantities may compensate each other.
e. The temperature coefficient of the specific
resistance ,has very different values for different
materials; it is for example positive and about
0.4 per cent per degree eentigrade for copper,
platinum and 'iron, and practically zero for constantan and manganin. If e is positive the resistance
increases with increasing current through the wire,
so that with a high current relatively too much
energy is developed. The deflection of the galvanometer will thus be greater than proportional to the
. square of the primary current.
Ä. The temperature coefficient of heat conductivity
is much smaller than that of electrical resistance;
it is weakly negative for the pure metals and
positive for constantan and manganin. Experience
has shown' that its înfluence is slight; the reason
for this lies in the above-mentioned fact that a
small value of Ä 0Ii. the one hand improves the heat
insulation of the filament, and on' the: other hand
it prevents the conduction of heat to the thermocouple. A change in Ä therefore will 'in the first
approximation
only influence the variation of
temperature
along the filament, but not the
temperature öf the middle. This does not of course
hold for the loss of heat through the thermocouple,
which is proportional to the heat conductivity of
the thermocouple. If the latter has a positive
temperature
coefficient the result is that the
deflection increases more slowly' than proportional
to the square of the, current.
(J. The
Stefan-Boltzmann
law holds for the
heat radiation of a black body. According to this
iaw the amount of energy radiated per second
and per square centimetre ~f the body is proportional to the fourth power of the absolute
temperature.
If thé" radiation takes \ place in
surroundings at the temperaturè TO) the radiation
is then:
R = S (T4 _ T04)
2
cal'
,cm ·sec·degree4
.
If the temperature
differences under eonsideration
are not great, this expression may be replaced by:
R = S a (T - To) = S a ~.
The quantity a, however, now depends upon the
temperature difference, and as second app~oxjmation we may write:
a
a-r = ao
(1
+ a 1').
The constant a is approximately independent on the
temperature
difference, ,and between 200 C and
1000 C it is equal to
0.6 per cent per degree
Centigrade. The dependence. of <5 on température
results in the fact that the increase in temperature is
less than the formula indicates, and therefore the
deflection of the galvanometer changes less than
proportional to the square of the primary current.
In addition e, Ä and (J, the thermo-e.m.f. per
degree, the resistance of the thermocouple and
the heat resistance
from the middle of the
filament to the junction of the thermocouple may
also depend upon the temperature. In many cases
the temperature
coefficient of the thermo-e.m.f,
may be neglected. The resistance of the thermo- .
couple' forms merely' a part
of the resistance
of the meter circuit, ·so that its influ~nce may he
slight. It is very difficult to make an estimation of the heat resistance of the connection
between :filament and. thermocouple, and of the
way in which this depends upon the temperature;,
its influence becomes less according as the heat
resistance of the thermocouple is increased. With
poorly made junctions, however, this influence may
very well be feIt, so that the connection must be .
very carefully made.
From the above discussion it follows that .
particularly the radiation can cause great deviations
from the quadratic variation of the characteristic.
It is therefore desirable to make the radiation as
slight as possible. We have seen in the discussion
of the expression for -the température at the middle
of the filament that this temperature is independent
of the dimensions for a given resistance of the
:filament. The radiation, however, is 'proportional
to the surface of the wire; to make the radiation
small therefore the dimensions must be kept small. .
At small values of K the radiation constant' (J
no longer occurs in _the formulae. The value of K
is therefore a measure of the influence of radiation.
K is proportional to ljfiJ; the energy sensitivity
on the other hand is proportional to ljd2•
With constant energy sensitivity (ljd2 = Cl)'
therefore; Ct d3/2 must be made as small as possible;
d must therefore be small.
The same holds for the dimensions of the thermocouple. Wires with differe~t dimensions but with '
the same electrical resistance cause the sàme loss
of heat; if the dimensions, and therefore the surface,
are small, the. radiation has the. least influence.
Therefore
short
thin wires must be used
to, construct
thermojunctions
with
a
quadratic
characteristic.
By this means the
heat capacity of the thermo-couple is. decreased
+
,"
THERMOJUNCTIONS
JUNE 1938
at the same time, so that the final deflection of
the meter is more quickly attained. A limit is set
to the reduction of the dimensions, however,
because such reduction diminishes the ability to
withstand overloads.
171
for one minute. At the same time it is desirable
to know the current at which the instrument can
be used continuously. In practice it is then desirable
to use a still lower current in order to have a margin
of safety.
,.'
In the case of the Philips thermo-junctions
the current is indicated at which the thermoelectric force is 12 mV; in addition the maximum
We have seen t.hat the highest temperature of
'current
is given at which the instrument may
the filament does not occur in the middle of the
,be used continuously and the current at which
wire, but somewhere between the middle and the
.it is quickly burnt through. We may call a
ter~als.
If the ~urrent through the filament is
thermo-junction
which gives 12 mV at 10 mA,
made too high, the filament will burn through at
a 10 mA instrument.
those spots. It is therefore important that the
It is therefore only possible to speak of a thermoratio' of this maximum temperature to the temjunction for a definite maximum current; the
perature at the middle should' not be too great.
current at which the full deflection of the meter
A measure for this ratio, which is not easy to
is obtained depends upon both thermocouple and
calculate, is the ratio of the average temperature
meter.
of the wire vto the .temperature at the middle. For
The following must also be kept in mind. Suppose .
small values of K it is:
that we have a certain meter and the corresponding
2M
thermocouples for 10 mA and 20 mA. The resistance
2/3 - 4/15 K2
,1
(1/6 K -13/180 K3)
'of
the filaments, of these couples are respectively
.
raAuq
.
75 and 23' ohms. In a given circuit the resistance
I....!..5/12 K2
of the 10 mA couple may be too high, for instance
when
the couple must form part of a tuned circuit.
At a given value of K (thus at ,a given value of
In this case the resistance of the 20 mA couple is
l;.yd) the ratio becomes greater if d is decreased;
the dependence of d, however, is less, the smaller perhaps permissible. That couple may then he
the value of K. Further it is clear that a thin wire used if a more sensitive meter is available which
is more quickly damaged than a thick one at a gives the full deflection at 10 mA, when used with
given temperature; this is' a re~son for not using the 20 mA couple. In certain cases therefore a dial
instrument will not be used but 'a very sensitive ,
too thin a wire.
With large current thermo-junctions K is so mirror galvanometer, in order to be able to use a .
small. that the maximum temperature is practically . thermocouple withlowresistance for low currents also,
Overloading and sensitivity
+
equal to the temperature -at the middle. In this
Reversal effect
case the permissible current is not limited by the
The first thermo-junctions constructed exhibited'
melting of the wire, but by the overheating of the
a strong reversal effect, i.e. the deflection changed
cement with which the thermocouple is fastened
when the direction of the current of the primary .
to the filament. it is very difficult to find a cement
direct current was changed. There may be two
which is attacked only at a higher temperature
causes for this phenomènon:
than that which injures the filament:
~) One of these may be the Peltier
effect, which
. The overloading of thermo-junctions for weak
is the reverse of the thermo-electric effect. When
currents is therefore determined by the velocity
current is sent through a thermocouple the junction
of evaporation or the melting point of the filament,
becomes colder if, upon heating it, a current would
that of those for heavy currents by the properties
of the cement.
In direct relation to overloading is the indication
of the current for which the thermo-junction is,
intended. Since the degree of attack on the filament
~670F •
upon overloading depends on the length of time
Fig. 6. a) Peltier
effect in thermocouples. The junction
during which a given current is sent through the
of the couple lies on the filament and therefore
wire, it is impossible to indicate a maximum perpart of the heating current passes through it.
b) Reversal effect in thermocouples. The junction
missible current, if the time is not also indicated.
of the couple lies near the filament, but the heating
A suitable measure is for example the current at
current passes through a short section of one of
the wires of the thermo-couple,
.
which. the filament remains absolutely Undamaged
....
172
PHILIPS TECHNICAL REVIEW
have occurred in the same direction, and vice versa.
The Peltier
effect therefore occurs in the original
thermo-electric ammeter by Kl eme.n i é, and also
in' the case of the commonly used bridge circuits
which we shall not discuss here. With modern
thermocouples also the effect may appear when
the thermo-junction
is not electrically insulated
from the filament, and when it lies exactly on the
filament (see fig. 6a). In this type of construction
part of the primary current also passes through.
the junction of the thermocouple and gives rise
to the Peltier
effect. This may be avoided by
placing the junction near but not on the filament.
b) Even when the junction does not lie on the
filament a reversing effect mayalso appear (see
fig: 6b). At the point where filament and thermocouple touch each other, part ofthe primary current
will always pass through part of the thermocouple.
Due to the resistance of the latter a potential
difference will arise which leads to a small extra
current through the meter' whose direction is
reversed when the direction of the primary current
is reversed. In order to avoid this the thermocouple
must be insulatèd from the filament; when this has
been done the reversal effect will he found to
have disappeared and the thermocouple can he
calibrated with direct current.
ó
Vol. 3, No. 6
elaboration of this construction is formed by the
bridge arrangements in which a number of thermocouples are joined to form a square; the disadvantage
of this method is the presence of a strong Peltier
effect.
As was noted. above, the heat capacity also has
some influence on the speed of indication; therefore
small dimensions are desirable. The resistance of
the thermocouple mayalso
effect the speed. of
'indication because with too low resistance the
damping of the 'metre is considerable.
Dependence of the indication on frequency
The heat development in a wire is independent
of the frequency of the alternating current used for
heating up to very high frequencies. At very high
frequencies, however, the so-called skin effect
appears, and the resistance, and therefore the heat
development also, are greaterthan at low frequencies.
For the thickness of wire used the influence of
this effect is only appreciable at wave lengths of
less than one metre, so that it need not usually
be taken into account.
It is quite another question with the influences
of various capacities (see fig. 8). The capacity in
,,/
;~
Sjleed of indication
The ,final deflection is not rapidly reached
with a thermo-ammeter.
The main reason for
this is that a temperatjrre equilibrium must he
established in the filament and the thermocouple.
Since the thermocouple itself does not carry the
primary current, the heating up of the junction
'mu~t take place' from the filament by conduction,
for which some time is necessary. A type of construction is possible in which the thermocouple
alsó serves as filament (see fig.7); in this case
the heat is developed in every, element of' volume
of the thermocouple, so that only 'a small amount
of heat needs to be transported by conduction,
and the 'final deflection is quickly attained. An
'/
F'nn
-IU'"
uu
26.704
Fig. a:Diagram of a thermocouple withthe
capacities.
.
2670S
various parasitic
parallel. with the filament is the cause of part
of the current to be measured not passing
through the filament, and the' result is that
the deflection of the, metre is' too small. This
. is sometimes compensated by the self-inductiön of
tlfe filament. The capacity between the supply
leads has the same effect. There is further capacity
between the filament and the thermocouple which
is connected with earth capacitatively through the
galvanometer. This capacity is further increased by
the fact that the leads of the filament and those
of the thermocouple are close together in the pinch
0
Fig. 7. Thermocouple with direct heating. The condenser is
introduced so that the direct voltage' given by the couple
may not be short-circuited by the source of alternating current.
The self-inductance is introduced 'to prevent the alternating
. current from passing through the direct current metre.
..:liluu
JUNE
1938
THERMOJUNCTIONS
of the bulb and in the base. This capacity may be
partially reduced by using thermocouples without
bases in measurements at very high frequencies.
The third kind of capacitive influence, the capacity
of the leads to earth, is hereby also very much
reduced. In order to overcome the last-mentioned
influence it is absolutely essential that one side
of the filament be earthed.
The influence of these parasitic capacities becomes
greater the higher the resistance of the filament.
A thermocouple with a filament having a resistance
of 20 ohms gives reliable indications at a wave
length of 6 metres within 1 per cent; when the
thermocouple is used without a base the indication
is still reliable within 1 per cent at wave lengths
of 3 meters.
173
d) The delicate part ofthe measuring arrangement,
the thermocouple itself, is easily renewable; in
this connection the quadratic, or at least reliably
constant, characteristic is also important.
Survey of the Philips thermojunctions
The P hili ps thermo-electric meters are constructed with the above considerations in view.
Fig. 9 is a photograph of a fully assembled thermocouple and jig. 10 shows the interior of such an
instrument. The cover serves not only for decoration, but also for shielding the instrument from
heat radiation from the surroundings.
Advantages in the use of thermc-ammeters
In conclusion we give the following list of the
advantages connected with the use ofthermocouples
for measuring alternating current.
.
a) The effective value of the alternating current
is measured because use is made of the development
of heat, and therefore of a mechanism which varies
essentially with the square of the current. For this
reason the measurement is very little affected by
the presence of higher harmonics in the current
measured, and it is not at all affected by the phase
of these harmonics. This is a common property of
all thermal instruments.
b) Up to a very high frequency the indication
is independent of the frequency of the current
measured.
c) The energy consumption is small compared
with that of dynamometers and soft-iron instruments.
Fig. 10. Interior
The table gives a survey of the various properties
of the instruments. With all types the temperature
coefficient of the resistance of the filament is small;
for most measurements it may be neglected. The
indication is practically quadratic up to half
the filament current, where the thermo-electric
force is 12 mV; the deviation from the quadratic
varia tion is 2 per cent at the most in this range,
the thermo-electromotive force is about 4 mV at
that current, so 'that a full deflection can still be
obtained with sensitive dial instruments.
The
thermocouple is insulated electrically from the
filament; the instruments can therefore be calibrated
with direct current.
Type
number
assembled.
12 mV
EMF
at
approx.
mA
Th3
10
20
40
Th4
100
Th5
200
Thl
Th2
Fig. 9. Thermocouple
of a thermocouple.
resistance
of the
filament
ohm
75
23
7.5
2.2
1.1
resistance
of the
thermocouple
5
3
3
3
3
continuous use
up to
mA
maximum
permissible
current
mA
150
20
40
100
200
300
350
15
30
75
174
PHILIPS
TECHNICAL REVIEW
Vol. 3, No. 6
ASSEMBLY OF WIRELESS RECEIVERS
The photograph shows two conveyor belts at which radio receivers are being assembled. The belts
convey parts from one worker to the next. The girl takes the arriving piece of work from the belt,
performs the necessary operations which consist in the mounting of parts, taken from the stock at her
side, into the chassis by welding, soldering or screwing. For a smoothly running process the girl should
just have finished her task when the next piece of work arrives. In this way the chassis is gradually
assembled and at the end of the belt it can be mounted in its cabinet.