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Thermal electrodes based on “peltier effect”
132
ELECTROENCEPHALOGRAPHY AND CLINICAL NEUROPttYSIOLOGY
TECHNICAL
NOTES
THERMAL ELECTRODES BASED ON "PELTIER EFFECT" 1 D. G. STUART,PH.D. 2, L. H. OTT, PH.D. s AND F, C. CHESHIRE1 Department of Anatomy, School of Medicine, University of California at Los Angeles; Medical Research Programs, Veterans Administration Hospital, Long Beach; and Hughes Aircraft Company, Culver City, Calif. (U.S.A.) (Received for publication : October 4, 1961 ) This note describes the theory, construction and performance of solid state thermal electrodes that permit a fine control of the rate of w a r m i n g a n d cooling of small regions of the brain, skin and other organs. W h e n a direct current is passed between two dissimilar conductors there is a transfer of energy in the form of heat between their junction a n d the e n v i r o n m e n t . Depending on the direction of current flow, heat is either absorbed or lost at the junction, the reverse effect occurring at the opposite poles of the c o n d u c t o r s ("Peltier effect'", G o l d s m i d 1960; G r a y 1960; Ott 1962). In couples formed between m o s t conductors, the Joule heat losses overwhelm "Peltier effect" cooling. T h u s the effect h a d little biomedical use until quite recent advances in the field of semiconductor thermoelectric materials (loffe 1957). To date this h a s culminated in the development of b i s m u t h telturide alloys of two basic types: N, with an excess of negative carriers or electrons a n d P, with an excess o f positive carriers ov "holes". A flow of current across a j u n c t i o n from N type to P type alloy creates a "Peltier effect" of sufficient m a g n i t u d e to counteract Joule and conductive heat losses a n d hence produce substantial cooling. W h e n polarity is reversed and current flows f r o m P type to N type alloy, the rate a n d extent of w a r m i n g are considerably greater because the "Peltier effect" is also reversed and s u m m a t e s with Joule heat losses. A junction between these alloys can be placed in direct contact with the cortex a n d skin to evoke rapid thermal changes. However, use of this m e t h o d for subcortical cooling m a y not be feasible at present for t h r e e reasons. First, the alloys are too brittle to permit cutting to a crosssection less t h a n I m m >: 1 ram. The m i n i m u m d i m e n s i o n s of a n electrode c o m p o s e d of the N a n d P alloy insulated but for their junction, is over 2.5 m m > 2.0 ram. Second, when the junction is cold, the alloy 1-3 m m away is hot a ~ ! an electrode inserted into the brain would evoke two sui .:ortical stimuli, one cold and the other hot, within a few m m of each other. Third, in electrodes m a d e of alloys of very small diameter, the increased resistance to current t Presented as a paper to the A m e r i c a n Physiological Society, Bloomington, Indiana, September 1961. Medical Research Fellow, B a n k of A m e r i c a - G i a n nini F o u n t k :ion. :~ Senior Staff Engineer, Aerospace G r o u p , H u g h e s Aircraft C o m p a n y . -~ Principal Electronics Technician, D e p a r t m e n t of A n a t o m y , U.C.L.A.
flow a u g m e n t s Joule heat losses but not "Peltier effect" cooling. F o r these reasons suhcortical cooling is better achieved by connecting a silver wire to a junction and inserting only the former into the brain. METHODS
Construction T h e alloys are commercially] available ("Neelium'" thermoelectric alloy, types N and P, General Thermoelectric Corp., Princeton, New Jersey). Fig. 1 illustrates three types of t h e r m o d e which we have proved practical for stimulation of the cortex, subcortical structures and the skin. T h e r m o d e A is the simplest and consists of two bars of alloy, each with dimensions 25 m m > 4.5 m m : / 2.5 ram. To form a junction a circular copper base 1 m m thick and 1 cm in diameter was soldered to them. The solder, with a melting point of 142°C, is c o m p o s e d of 50 percent b i s m u t h , 47½ percent tin and 2½Per cent a n t i m o n y . An anodized a l u m i n u m cap was cemented to the copper with an epoxy resin. This cap, with cylindrical socket 6 m m deep and 6 m m in diameter, was fitted with a set screw to hold interchangeable probes. The alloys, opposite the base end (junction), were soldered to copper rods that can be attached to conventional stereotaxic a p p a r a t u s and which additionally serve to dissipate heat when the junction is to be m a d e cold. For this latter reason the rods are grooved to increase surface area. Encasing the lower end of the t h e r m o d e is a plastic phenolic tube, 1 m m thick, which serves to protect the brittle alloys. T h e r m o d e B is basically similar to the one described but a water cooler replaces the copper rods. Additionally, a plastic phenolic tube (not shown) encases four alloy couples, each the same size as the couple in thermode A, As s h o w n in the inset, current is passed serially from one couple to the next such that, when a cooling effect is required at the base of the thermode, current flows from N to P type alloy at the base and from P r o N at the opposite ends. T h u s , the couples are electrically in series but thermally in parallel. Eight junction effects are created, the four at the upper end being in reverse direction to those at the base. T h e r m o d e C is an extension of this concept in that the alloy couples are cascaded to create a greater temperature gradient. D u r i n g cooling, the two cold junctions in t h e r m a l contact with the a l u m i n u m cap have their hot opposite ends in contact with three cold j u n c t i o n s of the second layer of couples. The hot opposite ends of this second layer are in thermal contact with a water cooler.
Electroenceph. olin. NeurophysioL, 1962, 14 : 132 135
THERMAL ELECTRODES Not shown is the phenolic tube which encases these cascaded alloy couples. The probe labelled I consists of a copper cylinder to fill the aluminum socket so that the thermode presents a flat circular surface, 1 cm in diameter, for skin stimulation. The second probe is also a copper cylinder 18 mm in length and 6 mm in diameter designed to contact the surface of the brain for cortical stimulation. Probe III is best for subcortical stimulation and consists of an insulated silver wire, 0.75 mm thick, soldered to a cylindrical copper base. By gradually increasing the current from N to P type alloy in each thermode, the base is cooled to the peak
A.
133
in ten anesthetized cats and human skin. To evoke thermal changes in the cortex the probe touched the surface of the exposed suprasylvian gyrus and temperature was measured 7 mm deep to the surface of the gyrus. For subcortical warming and cooling a silver wire probe was stereotaxically lowered at a 45 ° angle to a depth 25 mm below the surface of the brain. Temperature changes were measured with a needle thermocouple that was stereotaxically inserted 2 mm dorsal to the tip of the probe. Rectal temperature was held constant throughout the duration of thermal stimulation. At the end of each cat experiment the brain was fixed in formalin, sectioned every 80 y and
THERMODES B. +
C.
,..,--~
~.-Aluminum
Cap
-- -Water Cooler
~CopperRod ~
/Phenolic Tube
$
.m..rc..
Glue Line
.CopperJunction
Glue Line
Base
[]
-Glue Line
-AluminumCap
l
~AA~'u ~ : u ~ ' ~ ~
Fig. 1 Schemata of three electrodes designed primarily for warming and cooling the brain. Polarity signs indicate cold junctions at the base of each thermode. Not drawn to scale. For explanation see text. temperature gradient between hot and cold ends of alloy couples. In this way it was determined that the optimum current for maximum temperature gradient was 5 A for thermode A, 3 for B, and 8 for C. Increasing the current beyond these values finally results in Joule heat losses (increasing as the square of the current) that overwhelm "Peltier effect" cooling (a linear function of the current). Reasonably flat d.c. is desirable; performance suffers if a.c. ripple exceeds 10 percent of peak value. The resistance of each thermode was less than 0.5 fL Thermodes A and B can be used without their cooling units, but with less efficiency. They have recently been adapted to unanesthetized animals. In this case cortical and subcortical probes were permanently attached to the calvarium of cats and the thermodes, without their cooling units, suspended by wires from the roof of a Faraday cage. Contact was made between thermode and probe as described above.
Performance tests The effectiveness of the thermodes was gauged by using them to warm and cool cortical and subcortical sites
alternate sections photographed. In this way the distance of the thermocouple from the thermode probe could be determined. During warming and cooling of human skin, a thermocouple was placed between the surface of the skin and the thermode. RESULTS Fig. 2, A, shows that a fine control of the rate and extent of thermal change was achieved by varying the intensity of current flow. Fig. 2, B, illustrates a fall in skin temperature from 31 to 19°C during 4 rain of cooling with thermode A, the temperature rising to 45 ° C after 1 min of rewarming. However, subcortical warming and cooling with this thermode could effect changes of only :~- I°C in 5 rain of cooling and 5 min of rewarming. In contrast, Fig. 2, C, shows that thermodes B and C were able to cool to a far greater extent. 15-20 rain after beginning current flow through thermode B, the peak cooling effect was reached. This temperature gradient could then be maintained indefinitely. With thermode C, peak cooling occurred 30-40 min after beginning current flow, at which time cortical and subcortical regions were cooled 2-3°C fur-
Electroenceph. clin. Neurophysiol., 1962, 14:132-135
D. G. STUART, L. H. OTT AND F. C. CHESHIRE
134 A.
B.
39
~ - -
1Amp
- - ~ -
3 ,llmp
~ - ~
5 Amp
40
38
31 36
~ 35
"~
~- 30 p-
34
\,'--,.. \
33
. . . . . . . . . . . . / /: /i
.__ "_../
32
31
tw
c
2
3
4
20
c
5
6
7
8
9
lc
I
i0
Time - Min
C.
Time
- Min
38 36
32
28 H 26 24 E "? 38 37 50
t wA , /
35 34
0
;
10 Time-
15
~
z,
Min
Fig. 2 A. Warming and cooling the cortex with thermode A using current of 1, 3 and 5 A. Temperature was measured 7 mm ventral to the dorsal surface of the cortex with which the thermode was in contact. B. Warming and cooling human skin with thermode A. Temperature was measured between the thermode probe and the skin with which it was in contact. C. Warming and cooling the cortex and subeortex with thermodes B and C. Cortical temperature was measured as in A. Subcortical temperature was measured 2 mm dorsal to the tip of the thermode probe whic'a was angled at 45 ° to a hypothalamic site 25 mm ventral to the dorsal surface of the brain. C, onset of cooling by passing current from N type to P type alloy, t W, onset of warming by reversing polarity.
ther than was possible with thermode B. These thermodes changed the temperature of small skin regions from 31°C to 10-12°C after 5-7 rain of cooling. The results show that these electrodes are all suitable for warming and cooling the brain, skin and other organs, within a physiological range. However, B and C have cooling effects well beyond this range. They appear suitable
for both acute and chronic experimentation in a variety of laboratory animals. Ideally, such a thermode would operate with fine precision not only within 2-3 ° of normal body temperature but also be capable of cooling small cortical and subcortical regions to inactivity; normal function returning with rewarming. This would require the construction of a
Electroenceph. clin. Neurophysiol., 1962, 14:132--135
135
THERMAL ELECTRODES thermode capable of " p u m p i n g " more heat than was possible with those described in this note. Probably extension of the cascade principle illustrated in thermode C (Fig. 1), to a triple layer of couples with four rather than two junctions at the base would be sufficient to lower small cortical and subcortical regions to temperature below 20°C. The major difficulty in making a reversible subeortical lesion with a "Peltier" electrode is not the development of a very cold junction, but rather adequate thermal insulation of the probe to be inserted into the brain. Our best results have been with probes in which the silver wire was covered, except for 4 m m at the tip, with fiber glass encapsulated in a plastic tube. The outer dian~tcr of this probe was 1.5 mm, the silver being 0.75 mm thick. Even this overly large probe permitted heat exchange along the shaft, as well as along the uninsulated tip. SUMMARY Solid state "Peltier" electrodes have been constructed for thermal stimulation of the cortex, subeortex, skin and other organs. They are simple to operate and permit a
fine control of the rate and extent of warming and cooling within a limited temperature range. The thermodes discussed in thispaper were constructed in the laboratories of the Aerospace Group, Hughes Aircraft Company. The authors w i s h t o acknowledge this Company's cooperation. F o r interest and encouragement in the development of these thermodes we would like to thank Drs. E. Eldred and D. Maxwell of the Department of Anatomy, U.C.L.A. Medical Center. REFERENCES GOLDSMID, H. J. Applications of thermoelectricity. J. Wiley and Sons, New York, 1960, pp. 1-2. GRAY, P, E: The dynamic behavior of thermoelectric devices. Technology Press of M.I.T., Cambridge, Mass., 1960, pp. 1-3.
Io~E, A. F. Semiconductor thermoelements and thermoelectric cooling. Infosearch Ltd., London, 1957, pp. 1-184.
OTr, L. H. Thermoelectricity - - New uses for a n old principle. Radio Electronics, 1962, 23: 26-29.
Reference: STUART, D. G., OTT, L. H. and CHESHIRE, F.C. Thermal electrodes based on "Peltier effect". Electroence.ph, din. NeurophysioL, 1962, 14: 132-135.
ELECTROENCEPHALOGRAPHY AND CLINICAL NEUROPttYSIOLOGY
TECHNICAL
NOTES
THERMAL ELECTRODES BASED ON "PELTIER EFFECT" 1 D. G. STUART,PH.D. 2, L. H. OTT, PH.D. s AND F, C. CHESHIRE1 Department of Anatomy, School of Medicine, University of California at Los Angeles; Medical Research Programs, Veterans Administration Hospital, Long Beach; and Hughes Aircraft Company, Culver City, Calif. (U.S.A.) (Received for publication : October 4, 1961 ) This note describes the theory, construction and performance of solid state thermal electrodes that permit a fine control of the rate of w a r m i n g a n d cooling of small regions of the brain, skin and other organs. W h e n a direct current is passed between two dissimilar conductors there is a transfer of energy in the form of heat between their junction a n d the e n v i r o n m e n t . Depending on the direction of current flow, heat is either absorbed or lost at the junction, the reverse effect occurring at the opposite poles of the c o n d u c t o r s ("Peltier effect'", G o l d s m i d 1960; G r a y 1960; Ott 1962). In couples formed between m o s t conductors, the Joule heat losses overwhelm "Peltier effect" cooling. T h u s the effect h a d little biomedical use until quite recent advances in the field of semiconductor thermoelectric materials (loffe 1957). To date this h a s culminated in the development of b i s m u t h telturide alloys of two basic types: N, with an excess of negative carriers or electrons a n d P, with an excess o f positive carriers ov "holes". A flow of current across a j u n c t i o n from N type to P type alloy creates a "Peltier effect" of sufficient m a g n i t u d e to counteract Joule and conductive heat losses a n d hence produce substantial cooling. W h e n polarity is reversed and current flows f r o m P type to N type alloy, the rate a n d extent of w a r m i n g are considerably greater because the "Peltier effect" is also reversed and s u m m a t e s with Joule heat losses. A junction between these alloys can be placed in direct contact with the cortex a n d skin to evoke rapid thermal changes. However, use of this m e t h o d for subcortical cooling m a y not be feasible at present for t h r e e reasons. First, the alloys are too brittle to permit cutting to a crosssection less t h a n I m m >: 1 ram. The m i n i m u m d i m e n s i o n s of a n electrode c o m p o s e d of the N a n d P alloy insulated but for their junction, is over 2.5 m m > 2.0 ram. Second, when the junction is cold, the alloy 1-3 m m away is hot a ~ ! an electrode inserted into the brain would evoke two sui .:ortical stimuli, one cold and the other hot, within a few m m of each other. Third, in electrodes m a d e of alloys of very small diameter, the increased resistance to current t Presented as a paper to the A m e r i c a n Physiological Society, Bloomington, Indiana, September 1961. Medical Research Fellow, B a n k of A m e r i c a - G i a n nini F o u n t k :ion. :~ Senior Staff Engineer, Aerospace G r o u p , H u g h e s Aircraft C o m p a n y . -~ Principal Electronics Technician, D e p a r t m e n t of A n a t o m y , U.C.L.A.
flow a u g m e n t s Joule heat losses but not "Peltier effect" cooling. F o r these reasons suhcortical cooling is better achieved by connecting a silver wire to a junction and inserting only the former into the brain. METHODS
Construction T h e alloys are commercially] available ("Neelium'" thermoelectric alloy, types N and P, General Thermoelectric Corp., Princeton, New Jersey). Fig. 1 illustrates three types of t h e r m o d e which we have proved practical for stimulation of the cortex, subcortical structures and the skin. T h e r m o d e A is the simplest and consists of two bars of alloy, each with dimensions 25 m m > 4.5 m m : / 2.5 ram. To form a junction a circular copper base 1 m m thick and 1 cm in diameter was soldered to them. The solder, with a melting point of 142°C, is c o m p o s e d of 50 percent b i s m u t h , 47½ percent tin and 2½Per cent a n t i m o n y . An anodized a l u m i n u m cap was cemented to the copper with an epoxy resin. This cap, with cylindrical socket 6 m m deep and 6 m m in diameter, was fitted with a set screw to hold interchangeable probes. The alloys, opposite the base end (junction), were soldered to copper rods that can be attached to conventional stereotaxic a p p a r a t u s and which additionally serve to dissipate heat when the junction is to be m a d e cold. For this latter reason the rods are grooved to increase surface area. Encasing the lower end of the t h e r m o d e is a plastic phenolic tube, 1 m m thick, which serves to protect the brittle alloys. T h e r m o d e B is basically similar to the one described but a water cooler replaces the copper rods. Additionally, a plastic phenolic tube (not shown) encases four alloy couples, each the same size as the couple in thermode A, As s h o w n in the inset, current is passed serially from one couple to the next such that, when a cooling effect is required at the base of the thermode, current flows from N to P type alloy at the base and from P r o N at the opposite ends. T h u s , the couples are electrically in series but thermally in parallel. Eight junction effects are created, the four at the upper end being in reverse direction to those at the base. T h e r m o d e C is an extension of this concept in that the alloy couples are cascaded to create a greater temperature gradient. D u r i n g cooling, the two cold junctions in t h e r m a l contact with the a l u m i n u m cap have their hot opposite ends in contact with three cold j u n c t i o n s of the second layer of couples. The hot opposite ends of this second layer are in thermal contact with a water cooler.
Electroenceph. olin. NeurophysioL, 1962, 14 : 132 135
THERMAL ELECTRODES Not shown is the phenolic tube which encases these cascaded alloy couples. The probe labelled I consists of a copper cylinder to fill the aluminum socket so that the thermode presents a flat circular surface, 1 cm in diameter, for skin stimulation. The second probe is also a copper cylinder 18 mm in length and 6 mm in diameter designed to contact the surface of the brain for cortical stimulation. Probe III is best for subcortical stimulation and consists of an insulated silver wire, 0.75 mm thick, soldered to a cylindrical copper base. By gradually increasing the current from N to P type alloy in each thermode, the base is cooled to the peak
A.
133
in ten anesthetized cats and human skin. To evoke thermal changes in the cortex the probe touched the surface of the exposed suprasylvian gyrus and temperature was measured 7 mm deep to the surface of the gyrus. For subcortical warming and cooling a silver wire probe was stereotaxically lowered at a 45 ° angle to a depth 25 mm below the surface of the brain. Temperature changes were measured with a needle thermocouple that was stereotaxically inserted 2 mm dorsal to the tip of the probe. Rectal temperature was held constant throughout the duration of thermal stimulation. At the end of each cat experiment the brain was fixed in formalin, sectioned every 80 y and
THERMODES B. +
C.
,..,--~
~.-Aluminum
Cap
-- -Water Cooler
~CopperRod ~
/Phenolic Tube
$
.m..rc..
Glue Line
.CopperJunction
Glue Line
Base
[]
-Glue Line
-AluminumCap
l
~AA~'u ~ : u ~ ' ~ ~
Fig. 1 Schemata of three electrodes designed primarily for warming and cooling the brain. Polarity signs indicate cold junctions at the base of each thermode. Not drawn to scale. For explanation see text. temperature gradient between hot and cold ends of alloy couples. In this way it was determined that the optimum current for maximum temperature gradient was 5 A for thermode A, 3 for B, and 8 for C. Increasing the current beyond these values finally results in Joule heat losses (increasing as the square of the current) that overwhelm "Peltier effect" cooling (a linear function of the current). Reasonably flat d.c. is desirable; performance suffers if a.c. ripple exceeds 10 percent of peak value. The resistance of each thermode was less than 0.5 fL Thermodes A and B can be used without their cooling units, but with less efficiency. They have recently been adapted to unanesthetized animals. In this case cortical and subcortical probes were permanently attached to the calvarium of cats and the thermodes, without their cooling units, suspended by wires from the roof of a Faraday cage. Contact was made between thermode and probe as described above.
Performance tests The effectiveness of the thermodes was gauged by using them to warm and cool cortical and subcortical sites
alternate sections photographed. In this way the distance of the thermocouple from the thermode probe could be determined. During warming and cooling of human skin, a thermocouple was placed between the surface of the skin and the thermode. RESULTS Fig. 2, A, shows that a fine control of the rate and extent of thermal change was achieved by varying the intensity of current flow. Fig. 2, B, illustrates a fall in skin temperature from 31 to 19°C during 4 rain of cooling with thermode A, the temperature rising to 45 ° C after 1 min of rewarming. However, subcortical warming and cooling with this thermode could effect changes of only :~- I°C in 5 rain of cooling and 5 min of rewarming. In contrast, Fig. 2, C, shows that thermodes B and C were able to cool to a far greater extent. 15-20 rain after beginning current flow through thermode B, the peak cooling effect was reached. This temperature gradient could then be maintained indefinitely. With thermode C, peak cooling occurred 30-40 min after beginning current flow, at which time cortical and subcortical regions were cooled 2-3°C fur-
Electroenceph. clin. Neurophysiol., 1962, 14:132-135
D. G. STUART, L. H. OTT AND F. C. CHESHIRE
134 A.
B.
39
~ - -
1Amp
- - ~ -
3 ,llmp
~ - ~
5 Amp
40
38
31 36
~ 35
"~
~- 30 p-
34
\,'--,.. \
33
. . . . . . . . . . . . / /: /i
.__ "_../
32
31
tw
c
2
3
4
20
c
5
6
7
8
9
lc
I
i0
Time - Min
C.
Time
- Min
38 36
32
28 H 26 24 E "? 38 37 50
t wA , /
35 34
0
;
10 Time-
15
~
z,
Min
Fig. 2 A. Warming and cooling the cortex with thermode A using current of 1, 3 and 5 A. Temperature was measured 7 mm ventral to the dorsal surface of the cortex with which the thermode was in contact. B. Warming and cooling human skin with thermode A. Temperature was measured between the thermode probe and the skin with which it was in contact. C. Warming and cooling the cortex and subeortex with thermodes B and C. Cortical temperature was measured as in A. Subcortical temperature was measured 2 mm dorsal to the tip of the thermode probe whic'a was angled at 45 ° to a hypothalamic site 25 mm ventral to the dorsal surface of the brain. C, onset of cooling by passing current from N type to P type alloy, t W, onset of warming by reversing polarity.
ther than was possible with thermode B. These thermodes changed the temperature of small skin regions from 31°C to 10-12°C after 5-7 rain of cooling. The results show that these electrodes are all suitable for warming and cooling the brain, skin and other organs, within a physiological range. However, B and C have cooling effects well beyond this range. They appear suitable
for both acute and chronic experimentation in a variety of laboratory animals. Ideally, such a thermode would operate with fine precision not only within 2-3 ° of normal body temperature but also be capable of cooling small cortical and subcortical regions to inactivity; normal function returning with rewarming. This would require the construction of a
Electroenceph. clin. Neurophysiol., 1962, 14:132--135
135
THERMAL ELECTRODES thermode capable of " p u m p i n g " more heat than was possible with those described in this note. Probably extension of the cascade principle illustrated in thermode C (Fig. 1), to a triple layer of couples with four rather than two junctions at the base would be sufficient to lower small cortical and subcortical regions to temperature below 20°C. The major difficulty in making a reversible subeortical lesion with a "Peltier" electrode is not the development of a very cold junction, but rather adequate thermal insulation of the probe to be inserted into the brain. Our best results have been with probes in which the silver wire was covered, except for 4 m m at the tip, with fiber glass encapsulated in a plastic tube. The outer dian~tcr of this probe was 1.5 mm, the silver being 0.75 mm thick. Even this overly large probe permitted heat exchange along the shaft, as well as along the uninsulated tip. SUMMARY Solid state "Peltier" electrodes have been constructed for thermal stimulation of the cortex, subeortex, skin and other organs. They are simple to operate and permit a
fine control of the rate and extent of warming and cooling within a limited temperature range. The thermodes discussed in thispaper were constructed in the laboratories of the Aerospace Group, Hughes Aircraft Company. The authors w i s h t o acknowledge this Company's cooperation. F o r interest and encouragement in the development of these thermodes we would like to thank Drs. E. Eldred and D. Maxwell of the Department of Anatomy, U.C.L.A. Medical Center. REFERENCES GOLDSMID, H. J. Applications of thermoelectricity. J. Wiley and Sons, New York, 1960, pp. 1-2. GRAY, P, E: The dynamic behavior of thermoelectric devices. Technology Press of M.I.T., Cambridge, Mass., 1960, pp. 1-3.
Io~E, A. F. Semiconductor thermoelements and thermoelectric cooling. Infosearch Ltd., London, 1957, pp. 1-184.
OTr, L. H. Thermoelectricity - - New uses for a n old principle. Radio Electronics, 1962, 23: 26-29.
Reference: STUART, D. G., OTT, L. H. and CHESHIRE, F.C. Thermal electrodes based on "Peltier effect". Electroence.ph, din. NeurophysioL, 1962, 14: 132-135.