Dodd, Jack G.

English

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Journal of the Arkansas Academy of ScienceVolume 11 Article 101958Simple Themistor Bridge for Absorbed RadiationMeasurementJack G. DoddDrury UniversityFollow this and additional works at: http://scholarworks.uark.edu/jaasPart of the Optics CommonsThis article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able toread, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking priorpermission from the publisher or the author.This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academyof Science by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu, ccmiddle@uark.edu.Recommended CitationDodd, Jack G. (1958) "Simple Themistor Bridge for Absorbed Radiation Measurement," Journal of the Arkansas Academy of Science:Vol. 11 , Article 10.Available at: http://scholarworks.uark.edu/jaas/vol11/iss1/1060A SIMPLE THERMISTOR BRIDGE FORABSORBED RADIATION MEASUREMENTJack G, DoddDrury CollegeGENERALAn investigation currently underway of the quan-tum efficiency of photolysis of sodium azide neces-sitated accurate measurement of absorbed radiationin the ultraviolet region of the spectrum. Sinceour small grating monochromot or coupled with a car-bon arc source delivered less than a hundred micro-watts at the output slit, and since onlya fractionof this energy may be absorbed by the sample, anyradiation detector used must be very sensitive.Simple thermocouples were out of the question, andthermopiles which could double as sample— holderswere very inconvenient to manufacture ,besides be-ing very fragile.For these reasons some sort of bolometric tech-nique was indicated. The characteristics of therm-istors made them admirably suited for this sort ofuse.( 1,2)DESIGN AND PERFORMANCE OF THE PHOTOLYSIS CELLSince the measurement of the ra te of pho tolysisof the inorganic azides was to be accomplished byme a suring the rate of evolution of nitrogen bymeansof the change in pressure ina continuously pumpedevacuated enclosure, the unit was constructed ofglass with a cemented quartz window through whichthe monochroma tor beam was to pass. The arrange-ment is shown in Figure 1.The sample-holder consisted of a piece of 1-milplatinum foilabout 4 x 20 mm spot - welded to a tung-sten lead sealed into a 19/38 standard taper. Twoother tungsten leads which serve as the other twoleads of the thermistor bridge were also sealed intothe taper as shown in the drawing.The platinum sample— holder served as one leadof the sensitive bridge thermistor. The other leadof the sample-holder thermistor was common to thebalancing thermistor which was connected between60Journal of the Arkansas Academy of Science, Vol. 11 [1958], Art. 10Published by Arkansas Academy of Science, 195861A SIMPLE THERMISTOR BRIDGE61Journal of the Arkansas Academy of Science, Vol. 11 [1958], Art. 10http://scholarworks.uark.edu/jaas/vol11/iss1/10ARKANSAS ACADEMY OF SCIENCE62the other two tungsten leads sealed into the taper.The second thermistor was positioned so that it-wasshaftowod from the incident light by the sample-holder and thus did not change temperature duringirradiation (see Figure 2).Because thermistors of the special physicalcharacteristics required were not available, sometime was devoted to the development of suitablethermistor ma terials . ( 3 )Two general methods were found suitable: In thefirst tried, aborax bead was fused onto a nichromeloop in a hot gas flame and alternately dipped inferrous sulfide powder and re— fused until a ther-mistor made from this material showed the desiredcold resistance. In this case, a cold resistanceof about 5^,000 ohms was used, although itwas foundrelatively easy to make thermistors having a coldresistance of from 25 ohms up.In the second method, a bead of soft glass wasfused onto a nichrome loop and alternately dippedinto ferric chloride and re-fused until ithad thedesired characteristics. This second method pro-duced the best thermistors. The temperature co-efficient of resistivity of thermistors producedby either method ranged from -.02 to-.03 per °C.After suitable thermistor material was made, adrop of it was fused onto the back of a platinumsample-holder with a fine platinum wire imbeddedin it. This formed the radiation sensitive junc-tion. Small adjustments in the cold resistance ofthis junction could be effected by varying the spac-ing between the imbedded platinum wire and the sam-ple-holder proper. This value was ad justed to about50,000 ohms, A thermistor of the same cold resis-tance was then mounted behind the sample-holder aspreviously described to form the ambient tempera-ture compensating element of the bridge.Since the maximum temperature change of thesample-holder was expected from calculation to beno more than ,05°C, and since no highly sensitivegalvanometer was available for use as a null in-strument, it was anticipated that the bridge out-put would have to be amplified to be useable. Asimple d.c. amplifier was therefore constructed.A schematic diagram of the instrument is shown inFigure 2. It had a voltage gain of about l6, andan output impedance of about 200 ohms.62Journal of the Arkansas Academy of Science, Vol. 11 [1958], Art. 10Published by Arkansas Academy of Science, 195863Journal of the Arkansas Academy of Science, Vol. 11 [1958], Art. 10http://scholarworks.uark.edu/jaas/vol11/iss1/10ARKANSAS ACADEMY OF SCIENCE64Using a helipot as the two external arras of thebridge, this amplifier and a 100 microampere, 100millivoltD'Arsonval galvanometer, a sensitivityto incident radiation upon the sample-holder of better than one galvanometer division per micro-wattof incident radiation was achieved.The incident radiation from the monochrorna tor¦was allowed to fall on the sample for one minute,and the galvanometer deflections noted. Assumingthat the ins trument was linear for these small tem-perature changes, the deflection was then propor-tional to the power absorbed by the sample.Since a certain amount of energy is released bythe photolytic decomposition of the sample, the in-dicated quantum efficiency should be too high; how-ever, this effect is expected to be small since thequantum ef ficiency o f the azides for photolysis isusually much less than unity.PRELIMINARY RESULTSThe sample-holder was blackened by evaporatingaluminum at a pressure of about 5 mm Hg to obtaina film of aluminum black, which is known to be agood absorber of ultraviolet radiation. Assumingthis to be a black body, the effective spectralemissivity of the sample may be readily calculatedfor any incident wave length.Since preliminary data taken by F. Koperski ofthis project on the reflectivity of solid azidesis now available, the calculated spectral emissiv-ity of the samples used in the photolysis work canbe used as an independent check on this method.Although the bridge should theoretica llybe in-dependent of ambient temperature changes, this isnot found to be so in practice, and lack of effec-tive ambient temperature control has prevented,so far, the taking of data.LITERATURE CITED1) Prigent, J., 1949. £. Fhys. Rad. 10;^8-64.2) Fuchslocher and Glazer" 1^54. Z. fur Phys.38:363-371.(3) Zwikker, 1954. "Physical Properties of SolidMaterials," Pergamon Press (London), p. 224.64Journal of the Arkansas Academy of Science, Vol. 11 [1958], Art. 10Published by Arkansas Academy of Science, 1958

Simple Themistor Bridge for Absorbed Radiation Measurement

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Journal of the Arkansas Academy of Science Volume 11 Article 10 1958 Simple Themistor Bridge for Absorbed Radiation Measurement Jack G. Dodd Drury University Follow this and additional works at: https://scholarworks.uark.edu/jaas  Part of the Optics Commons Recommended Citation Dodd, Jack G. (1958) "Simple Themistor Bridge for Absorbed Radiation Measurement," Journal of the Arkansas Academy of Science: Vol. 11 , Article 10. Available at: https://scholarworks.uark.edu/jaas/vol11/iss1/10 This article is available for use under the Creative Commons license: Attribution-NoDerivatives 4.0 International (CC BY-ND 4.0). Users are able to read, download, copy, print, distribute, search, link to the full texts of these articles, or use them for any other lawful purpose, without asking prior permission from the publisher or the author. This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Journal of the Arkansas Academy of Science by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu. 60A SIMPLE THERMISTOR BRIDGE FORABSORBED RADIATION MEASUREMENTJack G, DoddDrury CollegeGENERALAn investigation currently underway of the quan-tum efficiency of photolysis of sodium azide neces-sitated accurate measurement of absorbed radiationin the ultraviolet region of the spectrum. Sinceour small grating monochromot or coupled with a car-bon arc source delivered less than a hundred micro-watts at the output slit, and since onlya fractionof this energy may be absorbed by the sample, anyradiation detector used must be very sensitive.Simple thermocouples were out of the question, andthermopiles which could double as sample— holderswere very inconvenient to manufacture ,besides be-ing very fragile.For these reasons some sort of bolometric tech-nique was indicated. The characteristics of therm-istors made them admirably suited for this sort ofuse.( 1,2)DESIGN AND PERFORMANCE OF THE PHOTOLYSIS CELLSince the measurement of the ra te of pho tolysisof the inorganic azides was to be accomplished byme a suring the rate of evolution of nitrogen bymeansof the change in pressure ina continuously pumpedevacuated enclosure, the unit was constructed ofglass with a cemented quartz window through whichthe monochroma tor beam was to pass. The arrange-ment is shown in Figure 1.The sample-holder consisted of a piece of 1-milplatinum foilabout 4 x 20 mm spot-welded to a tung-sten lead sealed into a 19/38 standard taper. Twoother tungsten leads which serve as the other twoleads of the thermistor bridge were also sealed intothe taper as shown in the drawing.The platinum sample— holder served as one leadof the sensitive bridge thermistor. The other leadof the sample-holder thermistor was common to thebalancing thermistor which was connected between60Journal of the Arkansas Academy of Science, Vol. 11 [1958], Art. 10Published by Arkansas Academy of Science, 195861A SIMPLE THERMISTOR BRIDGE61Journal of the Arkansas Academy of Science, Vol. 11 [1958], Art. 10https://scholarworks.uark.edu/jaas/vol11/iss1/10ARKANSAS ACADEMY OF SCIENCE62the other two tungsten leads sealed into the taper.The second thermistor was positioned so that it-wasshaftowod from the incident light by the sample-holder and thus did not change temperature duringirradiation (see Figure 2).Because thermistors of the special physicalcharacteristics required were not available, sometime was devoted to the development of suitablethermistor ma terials . ( 3 )Two general methods were found suitable: In thefirst tried, aborax bead was fused onto a nichromeloop in a hot gas flame and alternately dipped inferrous sulfide powder and re— fused until a ther-mistor made from this material showed the desiredcold resistance. In this case, a cold resistanceof about 5^,000 ohms was used, although itwas foundrelatively easy to make thermistors having a coldresistance of from 25 ohms up.In the second method, a bead of soft glass wasfused onto a nichrome loop and alternately dippedinto ferric chloride and re-fused until ithad thedesired characteristics. This second method pro-duced the best thermistors. The temperature co-efficient of resistivity of thermistors producedby either method ranged from -.02 to-.03 per °C.After suitable thermistor material was made, adrop of it was fused onto the back of a platinumsample-holder with a fine platinum wire imbeddedin it. This formed the radiation sensitive junc-tion. Small adjustments in the cold resistance ofthis junction could be effected by varying the spac-ing between the imbedded platinum wire and the sam-ple-holder proper. This value was ad justed to about50,000 ohms, A thermistor of the same cold resis-tance was then mounted behind the sample-holder aspreviously described to form the ambient tempera-ture compensating element of the bridge.Since the maximum temperature change of thesample-holder was expected from calculation to beno more than ,05°C, and since no highly sensitivegalvanometer was available for use as a null in-strument, it was anticipated that the bridge out-put would have to be amplified to be useable. Asimple d.c. amplifier was therefore constructed.A schematic diagram of the instrument is shown inFigure 2. It had a voltage gain of about l6, andan output impedance of about 200 ohms.62Journal of the Arkansas Academy of Science, Vol. 11 [1958], Art. 10Published by Arkansas Academy of Science, 195863Journal of the Arkansas Academy of Science, Vol. 11 [1958], Art. 10https://scholarworks.uark.edu/jaas/vol11/iss1/10ARKANSAS ACADEMY OF SCIENCE64Using a helipot as the two external arras of thebridge, this amplifier and a 100 microampere, 100millivoltD'Arsonval galvanometer, a sensitivityto incident radiation upon the sample-holder of better than one galvanometer division per micro-wattof incident radiation was achieved.The incident radiation from the monochrorna tor¦was allowed to fall on the sample for one minute,and the galvanometer deflections noted. Assumingthat the ins trument was linear for these small tem-perature changes, the deflection was then propor-tional to the power absorbed by the sample.Since a certain amount of energy is released bythe photolytic decomposition of the sample, the in-dicated quantum efficiency should be too high; how-ever, this effect is expected to be small since thequantum ef ficiency o f the azides for photolysis isusually much less than unity.PRELIMINARY RESULTSThe sample-holder was blackened by evaporatingaluminum at a pressure of about 5 mm Hg to obtaina film of aluminum black, which is known to be agood absorber of ultraviolet radiation. Assumingthis to be a black body, the effective spectralemissivity of the sample may be readily calculatedfor any incident wave length.Since preliminary data taken by F. Koperski ofthis project on the reflectivity of solid azidesis now available, the calculated spectral emissiv-ity of the samples used in the photolysis work canbe used as an independent check on this method.Although the bridge should theoretica llybe in-dependent of ambient temperature changes, this isnot found to be so in practice, and lack of effec-tive ambient temperature control has prevented,so far, the taking of data.LITERATURE CITED1) Prigent, J., 1949. £. Fhys. Rad. 10;^8-64.2) Fuchslocher and Glazer" 1^54. Z. fur Phys.38:363-371.(3) Zwikker, 1954. "Physical Properties of SolidMaterials," Pergamon Press (London), p. 224.64Journal of the Arkansas Academy of Science, Vol. 11 [1958], Art. 10Published by Arkansas Academy of Science, 1958

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Simple Themistor Bridge for Absorbed Radiation Measurement

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