Abstrict
An electric heater for heating a workpiece including at least one
heat generating wire which generates heat upon application of an
electric current thereto, and which has a diameter of not more than
1 mm. Each heat generating wire has a heating portion which is formed
by bending so as to at least partially define a heating space. The
workpiece to be heated by the electric heater is positioned relative
to the heating space, such that the workpiece is spaced apart from
the heating portion of the heat generating wire or wires.
Claims
What is claimed is:
1. An electric heater for heating a glass coating on a resistor
body of a resistor thereby forming a protective glass covering,
said resistor having first and second lead wires, each of which
is connected to respective opposite ends of said resistor body,
the electric heater comprising:
at least one heat generating wire which generates heat upon application
of an electric current thereto, each of said at least one heat generating
wire having a bent heating portion whose shape at least partially
defines a heating space, said heating portion having a length which
is smaller than a length of said glass coating so that only said
glass coating, except end portions thereof, is accommodated in said
heating space, said heating portion being further dimensioned so
that said glass coating is spaced apart from said at least one heat
generating wire;
a pair of terminal portions connected to opposite ends of said
heating portion, for connecting said heating portion to an external
power source; and
first and second heat radiating members contacting said first and
second lead wires, respectively, so that heat transferred to said
first and second lead wires from said glass coating and said resistor
body is radiated through said first and second heat radiating members.
2. An electric heater according to claim 1, wherein said heating
portion of said each heat generating wire takes the form of a cylindrical
coil having a plurality of cylindrical turns, said glass coating
of said resistor body being positioned in a cylindrical space defined
by said cylindrical coil.
3. An electric heater according to claim 1, wherein said heating
portion of said each heat generating wire takes the form of a rectangular
coil having a plurality of rectangular turns, said glass coating
of said resistor body being positioned in a rectangular space defined
by said rectangular coil.
4. An electric heater according to claim 1, wherein said at least
one heat generating wire consists of two heat generating wires,
said heating portions of one of said two heat generating wires cooperating
with said heating portion of the other heat generating wire to define
said heating spaced therebetween.
5. An electric heating according to claim 4, wherein said heating
portions of said two heat generating wires are movable toward and
away from each other.
6. The electric heater of claim 1, wherein each of said at least
one heat generating wire has a diameter of not more than 1 mm.
7. The electric heater of claim 1, wherein said first and second
heat radiating members consist of a pair of insulative tubes which
are disposed adjacent said pair of lead wires and extend along an
axial direction of said resistor body.
8. The electric heater of claim 1, wherein said pair of lead wires
is formed of a base metal.
9. An electric heater for heating a glass coating on a resistor
body of a resistor thereby forming a protective glass covering,
said resistor having first and second lead wires, each of which
is connected to respective opposite ends of said resistor body,
comprising:
at least one heat generating wire which generates heat upon application
of an electric current thereto, said heat generating wire having
a bent heating portion whose shape at least partially defines a
heating space, at least said glass coating being accommodated in
said heating space without contacting said heat generating wire,
so that said glass coating is fired by said heating portion; and
first and second adiabatic members disposed adjacent said heat
generating wire and around said first and second lead wires, respectively,
such that said first and second lead wires are substantially completely
insulated from the heat generated by said heating portion.
10. The electric heater of claim 9, wherein said heating portion
fires said first portion at temperatures ranging from 500.degree.-800.degree.
C.
11. The electric heater of claim 9, wherein said adiabatic member
consists of an insulative tube which is disposed completely around
said second portion and extends along an axial direction of said
resistor body.
12. The electric heater of claim 9, wherein said heat generating
wire has a diameter of not more than 1 mm.
13. The electric heater of claim 9, wherein said pair of lead wires
are formed of a base metal.
14. A method of heating a glass coating on a resistor body of a
resistor thereby forming a protective glass covering, said resistor
having first and second lead wires, each of which is connected to
respective opposite ends of said resistor body, the method comprising
the steps of:
forming at least one heat generating wire which generates heat
upon application of an electric current thereto, so that said heat
generating wire has a bent heating portion whose shape at least
partially defines a heating space, and so that said heating portion
has a length which is smaller than a length of said glass coating;
forming a pair of terminal portions at opposite ends of said heating
portion for connecting said heating portion to an external power
source;
forming a pair of heat radiating members near said heating portion;
positioning said resistor relative to said heating space such that
said glass coating is spaced apart from said heat generating wire,
only said glass coating except for end portions thereof is accommodated
in said heating space and said pair of lead wires are supported
by said heat radiating members;
applying an electric current to said heat generating wire such
that only said glass coating is fired by said heating portion, and
such that heat transferred to said lead wires from said glass coating
and said resistor body is radiated through said heat radiating members;
and
removing said resistor from said heating space.
15. The electric heater of claim 14, wherein said pair of lead
wires are formed of a base metal.
16. A method of heating a glass coating on a resistor body of a
resistor thereby forming a protective glass covering, said resistor
having a pair of lead wires, each of which is connected to respective
opposite ends of said resistor body, comprising the steps of:
forming at least one heat generating wire which generates heat
upon application of an electric current thereto, so that said heat
generating wire has a bent heating portion whose shape at least
partially defines a heating space;
forming a pair of adiabatic members near said heating portion;
positioning said resistor relative to said heating space such that
said glass coating is spaced apart from said heat generating wire
and said pair of lead wires are disposed within said adiabatic members;
applying an electric current to said heat generating wire such
that said adiabatic members substantially completely insulate said
lead wires from heat generated by said heating portion; and
removing said resistor from said heating space.
17. The electric heating of claim 16, wherein said pair of lead
wires are formed of a base metal.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to an electric heater
used for heating a workpiece of a small size, which has both a portion
which must be heated and a portion which must not be heated, and
a method of heating the workpiece by means of such an electric heater.
More particularly, the present invention is concerned with an electric
heater which is suitably used for firing a protective glass for
a platinum-film resistor used in a film element of an air-flow sensor,
for example.
2. Discussion of the Prior Art
A platinum-film resistor is generally used in a film element of
an air-flow sensor for detecting a flow rate of air stream. The
platinum-film resistor is produced by attaching a pair of lead wires
made of base metal, to opposite ends of a ceramic body having a
platinum layer formed thereon by plating.
In producing the platinum-film resistor, the platinum plating layer
formed on the ceramic body is secured by firing thereof to the ceramic
body, and the base metal lead wires are connected by firing thereof
to the platinum plating layer. Thereafter, the platinum plating
layer on the ceramic body is coated with a glass, and the applied
glass is fired, whereby a protective glass for protecting the platinum
plating layer is formed on the ceramic body. It is generally recognized
that the entire length of the platinum plating layer (ceramic body)
to which a glass coating is applied is extremely short, e.g., in
the neighborhood of 4-5 mm. Further, no means is known for effectively
heating only the platinum plating layer coated with the glass. Under
these situations, the firing of the glass has been conventionally
effected by using a tubular furnace, which is constructed such that
a heat generating element is wound around a ceramic tube made of
alumina, for example, and which is adapted to heat the whole platinum-film
resistor accommodated therein. Alternatively, the glass on platinum
plating layer is fired in a box-like furnace which has a box-like
body made of brick, on which a heat generating element formed of
SiC, for example, is provided.
However, when the firing of glass is effected by heating the whole
platinum-film resistor in the tubular or box-like furnace, it takes
a long time for the furnace to be heated or cooled to a desired
degree, since the furnace has a relatively large heat capacity.
Further, since the firing temperature of the glass is extremely
high, e.g., in the neighborhood of 800.degree. C., the temperature
in the furnace is maintained for a considerably long time above
the temperature at which the lead wires are oxidized, (usually 400.degree.
C. or higher). Therefore, if only the platinum plating layer on
the ceramic body is coated with a glass, and the glass is fired
in the furnace with the lead wires being exposed to the atmosphere
in the furnace, the lead wires undergo a change of color due to
oxidation, and are even torn off in the worst case. For preventing
the oxidation of the lead wires, nitrogen or hydrogen gas may be
introduced into the furnace, so that the furnace contains therein
a neutral or reducing atmosphere. In this case, however, the atmosphere
in the furnace is likely to decompose the glass on the platinum
plating layer, or give rise to bubbles in the glass. Thus, it is
found difficult to employ the method of controlling the atmosphere
in the furnace in the manner as described above.
In view of the above, the lead wires as well as the platinum plating
layer on the ceramic body are generally coated with a glass, so
that the glass coating formed on the lead wires prevents the lead
wires from being oxidized during firing of the glass. This method
requires the procedure of removing the fired glass covering the
lead wires, after the resistor is taken out from the furnace. Thus,
the method suffers from considerable reduction in the productivity
of the platinum-film resistor.
In the conventional method using the tubular or box-like furnace
which has a relatively large heat capacity, it is difficult to control
the temperature in the furnace, at a constant level in a medium
to low temperature range below about 800.degree. C. Namely, the
furnace is likely to suffer from an overshoot or ringing phenomenon,
deterioration in heat efficiency, and an increased firing time.
An electric heating device may be used instead of the furnace as
described above. However, the electric heating device is incapable
of uniformly heating the workpiece, thereby causing local variation
in the heating temperature of the workpiece. The electric heating
device also suffers from an extremely large heat capacity, as compared
with that of the platinum-film resistor.
SUMMARY OF THE INVENTION
The present invention was developed in light of the above situations.
It is therefore a first object of the present invention to provide
an electric heater for heating a workpiece of a small size, such
as a platinum-film resistor, which has both a portion that must
be heated and a portion that must not be heated, such that only
the portion which must be heated is more effectively and more rapidly
heated and cooled by the present electric heater, than by the conventional
heating means.
It is a second object of the invention to provide a method of heating
a workpiece by means of the electric heater as described above.
The first object may be achieved according to one aspect of the
present invention, which provides an electric heater for heating
a workpiece, comprising at least one heat generating wire which
generates heat upon application of an electric current thereto,
and which has a diameter of not more than 1 mm, each of the above-indicated
at least one heat generating wire having a heating portion which
is formed by bending so as to at least partially define a heating
space, the workpiece being positioned relative to the heating space
such that the workpiece is spaced apart from the heating portion
of the above-indicated at least one heat generating wire.
The electric heater constructed according to the present invention
is formed by bending the heat generating wire or wires so as to
define a suitable heating space therein. A workpiece to be heated
is positioned relative to the heating space such that the workpiece
is spaced apart from the heating portion of the heat generating
wire or wires. Since the heat generating wire has a diameter of
1 mm or smaller and accordingly, has an extremely small heat capacity,
the electric heater is capable of efficiently and rapidly heating
and cooling only that portion of the workpiece which must be heated,
even when the workpiece is small in size and has both that portion
which must be heated and a portion which must not be heated. Thus,
the present electric heater is able to heat only the portion of
the workpiece which must be heated, in a reduced time and with an
improved heat efficiency, according to a desired heating schedule,
without suffering from any inconvenience caused by heating of the
portion which must not be heated. Further, since the configuration
of the heating portion(s) of the heat generating wire(s), i.e.,
the shape of the heating space can be determined as desired, following
that of the portion of the workpiece which must be heated, that
portion of the workpiece ca be uniformly heated by the present electric
heater irrespective of the configuration of the workpiece.
In one form of the present invention, each of the above-indicated
at least one heat generating wire has a pair of terminal portions
connected to opposite ends of the heating portion, the terminal
portions being operatively connected to an external power source.
As described above, the workpiece may have a first portion which
must be heated and a second portion which must be prevented from
being heated. In this case, the heating space has dimensions which
are determined so that only the first portion of the workpiece is
accommodated in the heating space, so that the first portion is
exclusively heated by the heating portion of each heat generating
wire. According to one feature of this form of the invention, the
electric heater further comprises a heat radiating member for contacting
the second portion of the workpiece.
The heating portion of each heat generating wire may take the form
of a cylindrical coil having a plurality of cylindrical turns. In
this case, the first portion of the workpiece is positioned in a
cylindrical space defined by the cylindrical coil.
Alternatively, the heating portion of each heat generating wire
may take the form of a rectangular coil having a plurality of rectangular
turns. In this case, too, the first portion of the workpiece is
positioned in a rectangular space defined by the rectangular coil.
The above-indicated at least one heat generating wire may consist
of two heat generating wires. In this case, the heating portion
of one of the two heat generating wires cooperates with the heating
portion of the other heat generating wire to define the heating
space therebetween. The heating portions of the two heat generating
wires may be movable toward and away from each other.
The first object may also be achieved according to another aspect
of the present invention, which provides an electric heater for
heating a workpiece having a first portion which must be heated
and a second portion which must be prevented from being heated,
comprising (a) at least one heat generating wire which generates
heat upon application of an electric current thereto, and which
has a diameter of not more than 1 mm, each of the above-indicated
at least one heat generating wire having a heating portion which
is formed by bending so as to at least partially define a heating
space, at least the first portion being accommodated in the heating
space without contacting the above-indicated at least one heat generating
wire, so that the first portion is heated by the heating portion
of the heat generating wire or wires, and (b) an adiabatic member
for supporting the second portion so that the second portion is
insulated from the heat generated by the heating portion.
In the electric heater constructed as described above, the workpiece
is heated such that its second portion which must not be heated
is supported by the adiabatic member, and is thereby protected against
heat from the surrounding space thereof. In this arrangement, even
if the workpiece is heated by the heater while a part of the second
portion as well as the first portion is accommodated in the heating
space, only the first portion is effectively heated in a reduced
time and with an improved heat efficiency as described above.
The second object may be achieved according to a further aspect
of the invention, which provides a method of heating a workpiece
by means of an electric heater, comprising the steps of (a) forming
the electric heater from at least one heat generating wire having
a diameter of not more than 1 mm, so that each heat generating wire
has a heating portion which is formed by bending so as to at least
partially define a heating space, (b) positioning the workpiece
relative to the heating space such that the workpiece is spaced
apart from the above-indicated at least one heat generating wire,
and such that a portion of the workpiece is accommodated in the
heating space, and (c) applying an electric current to the above-indicated
at least one heat generating wire so that only the portion of the
workpiece is heated by the heating portion of the heat generating
wire or wires.
According to the above-described method, the heater is capable
of efficiently and exclusively heating that portion of the workpiece
which must be heated, even where the workpiece is small-sized and
has both that portion which must be heated and a portion which must
not be heated.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and optional objects, features and advantages of the
present invention will be better understood by reading the following
description of presently preferred embodiments of the invention,
when considered in conjunction with the accompanying drawings, in
which:
FIG. 1 is a view showing one embodiment of an electric heater according
to the present invention, together with a platinum-film resistor
as a workpiece to be heated by the heater;
FIG. 2 is a block diagram showing an example of a current supply
device for applying a controlled amount of electric current to the
heater of FIG. 1, so as to energize the heater to generate heat;
FIG. 3 is a graph showing a schedule of heat treatment by way of
example, when the electric heater of FIG. 1 is used for firing a
protective glass applied to the platinum-film resistor;
FIG. 4 is a view showing another embodiment of the electric heater
of the invention, together with a platinum-film resistor as a workpiece
to be heated by the heater; and
FIGS. 5, 6 and 7 are perspective views showing further embodiments
of the electric heater according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1, there is shown one preferred embodiment
of the present invention in the form of an electric heater 18, together
with a platinum-film resistor 10 which is used as a workpiece to
be heated. The platinum-film resistor 10 consists of a resistor
body 12 as a first portion which must be heated, and a pair of lead
wires 16, 16 as a second portion which must not be heated. The resistor
body 12 is formed from a ceramic tube made of alumina, for example.
The ceramic tube is plated with platinum so that a platinum layer
having a predetermined thickness is formed on the circumferential
surface thereof, and then suitably heat-treated to provide the platinum-plated
resistor body 12. The lead wires 16, 16 are made of base metal,
and are connected to axially opposite ends of the tubular resistor
body 12, by means of a pair of platinum lugs 14, 14 which are formed
by firing platinum paste applied to the axially opposite end faces
of the resistor body 12.
The electric heater 18 constructed according to the present invention
is used for firing a protective glass which covers the resistor
body 12 of the platinum-film resistor 10. The heater 18 is constituted
by a heat generating wire, which is made of a suitable material
such as platinum, Ni-Cr alloy, and Fe-Cr alloy, that generates heat
upon energization thereof. The heat generating wire has a diameter
of 1 mm or smaller, preferably, 0.5 mm or smaller.
The electric heater 18 in the form of the heat generating wire
as described above consists of a heating portion in the form of
a cylindrical coil portion 18a, and two terminal portions 18b, 18b
connected to opposite ends of the cylindrical coil portion 18a.
The cylindrical coil portion 18a has a diameter a little larger
than the outside diameter of the tubular resistor body 12 of the
resistor 10, and an axial length a little shorter than that of the
resistor body 12. This coil portion 18a is wound around the resistor
body 12 at a predetermined pitch, with a suitable spacing left therebetween,
as shown in FIG. 1.
In the instant embodiment, the electric heater 18 is adapted to
fire the glass applied to the surface of the resistor body 12 of
the resistor 10. For this purpose, the platinum-film resistor 10
is inserted through an internal cylindrical space of the cylindrical
coil portion 18a of the heater 18, so that the resistor body 12
is placed in substantially coaxial relationship with the coil portion
18a, as shown in FIG. 1. In other words, the resistor body 12 is
positioned at a radially central portion of the above-indicated
internal cylindrical space, i.e., cylindrical heating space, which
is surrounded by the heat generating wire of the coil portion 18a
of the heater 18, such that the resistor body 12 stands apart from
the coil portion 18a. It will be understood that the platinum-film
resistor 10 as a workpiece to be heated is positioned relative to
the heater 18 so that only the resistor body 12 as the above-indicated
first portion is heated by the heating coil portion 18a of the heater
18.
With the platinum-film resistor 10 being positioned relative to
the electric heater 18 as described above, an electric current is
applied to the heater 18, according to a desired schedule of heat
treatment. Such energization of the heater 18 is effected preferably
with the lead wires 16, 16 of the resistor 10 being held in contact
with respective heat radiating rods 20, 20, as shown in FIG. 1.
More preferably, the heater 18 is energized while a semi-cylindrical
covering member (not shown) is disposed above the coil portion 18a
of the heater 18, with a relatively small spacing left therebetween.
Between the electric heater 18, more precisely, the terminal portions
18b, 18b, and a power source 22, there is interposed a control circuit
24 which includes a relay or a thyristor, for example. In this arrangement,
a suitable amount of electric current is applied to the heater 18
by operation of the control circuit 24 under control of a programmable
controller 26 connected to the circuit 24. If the power source 22
is adapted to supply a constant voltage or current, the programmable
controller 26 is also used to directly control the power source
22 for supply of the constant voltage or current, as indicated by
a broken line in FIG. 2.
As described above, the electric heater 18 is disposed such that
only the resistor body 12 of the platinum-film resistor 10 is surrounded
by the coil portion 18a of the heater 18. Further, the heater 18
is formed by the heat generating wire whose diameter is 1 mm or
smaller, and which has a considerably small heat capacity. Therefore,
the heating temperature of the heater 81 is efficiently or rapidly
raised or lowered according to a change in the amount of electric
current applied to the heater 18. It will be understood from the
above description that the instant electric heater 18 is capable
of exclusively heating the selected portion (resistor body 12) of
the platinum-film resistor 10, precisely according to the predetermined
schedule of heat treatment. It follows that the instant electric
heater 18 is able to effectively fire the glass applied to the surface
of the resistor body 12 so as to form the protective glass thereon,
without causing unfavorable oxidation of the lead wires 16, 16 disposed
adjacent to the resistor body 12.
In the manner as described above, the protective glass applied
to the platinum-plated surface of the resistor body 12 can be fired
simply by inserting the platinum-film resistor 10 through the cylindrical
space of the heating coil portion 18a of the heater 18, and applying
to the heater 18 the electric current controlled by the control
circuit 24. The heater 18 does not cause unfavorable oxidation of
the lead wires 16, 16. This means that the procedure for forming
the protective glass on the resistor body 12 can be considerably
simplified, and does not require applying a glass also to the lead
wires 16, 16 for protection against oxidation of the wires during
the firing operation, and removing the applied glass from the lead
wires 16, 16 after the firing operation. Further, since the heater
18 having a considerably small heat capacity can be rapidly heated
or cooled, the heater 18 is advantageous for its remarkably improved
heat efficiency, assuring a considerably reduced time required for
firing the glass on the resistor body 18. Moreover, the present
electric heater 18 is free from an overshoot or ringing phenomenon,
even when the heater 18 is controlled such that the heating temperature
of the heater 18 is kept constant in a medium to low temperature
range, more specifically, around 800.degree. C. or lower.
Since the instant electric heater 18 itself is considerably compact
or small-sized, a space for accommodating the heater 18 can be relatively
small, when operated under vacuum, or in a nitrogen or hydrogen
atmosphere.
As described above, the firing of the glass on the resistor body
12 is preferably effected with the heat radiation rods 20, 20 being
held in contact with the lead wires 16, 16, which constitute the
above-indicated second portion of the platinum-film resistor 10,
i.e., the portion which must not be heated by the heater 18. This
arrangement avoids an increase in the temperature of the lead wires
16, 16 during firing of the glass, thereby effectively preventing
the lead wires 16, 16 from being oxidized. Further, the semi-cylindrical
covering member may be disposed above the heating coil portion 18a
of the heater 18 such that the covering member is spaced a small
distance from the heater 18, as also described above. In this arrangement,
the covering member serves to prevent occurrence of ascending current,
whereby the resistor body 12 as the first portion which must be
heated can be uniformly heated with an improved heat efficiency.
An example of the platinum-film resistor 10 as a workpiece to be
heated was prepared in the following manner. At first, a ceramic
tube having a diameter of 2 mm and a length of 4 mm was plated with
platinum and then fired, to thereby form a platinum layer having
a thickness of 0.10 .mu.m, on the surface of the ceramic tube. Thus,
the resistor body 12 having a platinum-plated surface was prepared.
ON the other hand, the lead wires 16, 16 were prepared, each having
a diameter of 0.15 mm and a length of 10 mm and made of an Ni-Fe
alloy. Then, the lead wires 16, 16 were connected to the axially
opposite ends of the resistor body 12, by means of platinum lugs
14, 14 which were formed by firing platinum paste applied to the
opposite ends of the resistor body 12. A glass was applied to the
resistor body 12 of the platinum-film resistor 10 produced.
On the other hand, there was prepared an example of the electric
heater 18 as shown in FIG. 1, which is used for firing the protective
glass formed on the resistor body 12 of the platinum-film resistor
10. The electric heater 18 was formed by a platinum wire having
a diameter of 0.4 mm, such that the heating coil portion 18a of
the heater 18 has an axial length of 3 mm, and an inside diameter
a little larger than the diameter of the ceramic tube. This electric
heater 18 was positioned relative to the platinum-film resistor
10 as shown in FIG. 1, and then energized according to a heating
schedule as shown in FIG. 3, to generate heat for firing the glass
applied to the surface of the resistor body 12 of the resistor 10.
After firing of the protective glass on the resistor body 12, the
state of the fired glass and the degree of oxidation of the lead
wires 16, 16 were observed. The observation revealed that the state
of the glass after firing thereof was extremely good, and that the
lead wires 16, 16 were oxidized only at their proximal portions
which were not held in contact with the heat radiating rods 20,
20. Although the surfaces of the proximal end portions of the wires
16, 16 experienced a slight degree of discoloration, such oxidation
had substantially no influence on the operation of the lead wires
16, 16. Further, it was recognized that the temperature in the heating
space of the heater 18, i.e., in the internal cylindrical space
of the coil portion 18a, closely followed a predetermined change
in the heating temperature according to the heating schedule as
shown in FIG. 3. It was also recognized that the heater 18 was controlled
so as to maintain the heating temperature at 500.degree. C., without
the occurrence of overshoot or ringing.
As a comparative example, the heat treatment similar to that as
described above was conducted with respect to the platinum-film
resistor 10, without using the heat radiating rods 20, 20. The observation
after the heat treatment revealed that the length of a portion of
each lead wire 16 which experienced discoloration was increased
to 3 to 4 mm. However, these discolored portions of the lead wires
16, 16 were oxidized only at their surfaces, and will not cause
any problems in the practical use of the resistor 10.
Referring next to FIG. 4, there is shown another embodiment of
the present invention. This embodiment is different from the preceding
embodiment, only in the relationship between the lengths of the
resistor body 12 of the platinum-film resistor 10, and the coil
portion 18a of the electric heater 18.
More specifically, the heating coil portion 18a of the heater 18
of the present embodiment has a length which is a little larger
than that of the resistor body 12 of the resistor 10. Upon firing
of the glass applied to the surface of the resistor body 12, a portion
of the lead wires 16, 16 adjacent to the resistor body 12 as well
as the resistor body 12 is positioned within the heating space of
the heater 18 (internal space of the coil portion 18a). In other
words, the heating space of the heater 18 extends over a part of
the second portion 16 of the resistor 10, which must not be heater,
as well as the first portion 12 which must be heated.
In the instant embodiment, the lead wires 16, 16 of the resistor
10 are inserted into respective adiabatic pipes 28, 28 made of alumina,
for example, as shown in FIG. 4. In this arrangement, the protective
glass is fired by the heater 18 as described above while the lead
wires 16, 16 supported by the adiabatic pipes 28, 28 are protected
against heat from the surrounding space thereof. As a result, the
heater 18 of the instant embodiment is able to provide the similar
effects as provided in the preceding embodiment.
It is possible to insert a heat radiating rod into each of the
adiabatic pipes 28, 28, such that the heat radiating rod is held
in contact with the corresponding lead wire 16 during the firing
operation of the heater 18. This arrangement effectively avoids
an increase in the temperature of the lead wires 16, 16, and accordingly
prevents the oxidation of the wires 16, 16.
It is also possible to water-cool the adiabatic pipes 28, 28, for
preventing the oxidation of the lead wires 16, 16.
While the present invention has been described in detail in its
presently preferred embodiments referring to the accompanying drawings,
it is to be understood that the invention is not construed to be
limited to the details of the illustrated embodiments but that the
invention may be embodied with various changes, modifications and
improvements, which may occur to those skilled in the art, without
departing from the spirit and scope of the invention as defined
in the appended claims.
While the electric heater 18 is formed by the heat generating wire
having a cylindrical coil with cylindrical turns in the illustrated
embodiments, the heater 18 may be formed by a heat generating wire
as shown in FIG. 5, which has a rectangular heating coil portion
with rectangular turns. It is also possible to use a plurality of
heater elements 30 which are formed by bending respective heat generating
wires in appropriate directions. The thus produced heater elements
30 are disposed to form the electric heater 18, as shown in FIGS.
6 and 7. These heater elements 30 are disposed movably toward and
away form each other, as necessary. With the heater elements 30
moved toward each other as indicated by arrows of FIG. 6 and 7,
the appropriate heating space of the heater 18 is formed between
the heating portions of the elements 30. This arrangement makes
it much easier to place the workpiece to be heated in the heating
space of the heater 18, which leads to simplification of the heating
procedure.
In the illustrated embodiments, the electric heater 18 is adapted
to heat the platinum-film resistor 10 as the workpiece to be heated.
However, it will be readily understood that the heater according
to the present invention may be used to heat at workpiece other
than the platinum-film resistor. In any case, the configuration
of the heat generating wire or wires of the heater may be suitably
determined depending upon the position of the above-indicated first
portion of the workpiece. In the case where the heat generating
wire is relatively long in light of its strength, and where the
heater cannot maintain its configuration on its own strength, a
portion of the heater may be supported by an adiabatic ceramic fiber
or fibers.
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