LECTURE-TABLE THERMOMETER AND VOLTMETER FREDERIC B. DUTTON Michigan State College, East Lansing, Michigan
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pamphlet entitled "Thermoelectric Thermometry" has recently become available.' The lecture-table thermometer consists of a thermocouple, a potentiometer, a converteramplifier, a motor and gear train, an indicator, and lamp and scale, s h o m in Figures 1, 2, and 3.
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CIRCUIT
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Forc~round,t l m m o o o e ~ l e s ; C, ten-junction thermocouple mounted for freeringmint-lowering measurements: A , alternate acales: B, extension cord for thermocouple or cell eonnootiona; extreme right, funnel for separating ice from aol~tiona.
The circuits are shown schematically in Figure 4. A three-volt battery is connected across the end of the 1000-ohm potentiometer R1 through switch 81. The moving potentiometer contact makes it possible to feed from 0 to 3 volts to the balance of the circuit. This voltage is measured by the voltmeter V . Two volts is the usual operating potential. One side of the battery circuit is connected to ~ ~ contact of ~ a five-rrosition ~ t the moving switch S3.
' DIKE,PAULH., "Thermoelectric Thermometry," available a t $1 from The Leeds and Northrup Co., Philadelphia, Pa. .
demonstrations involving temperature changes present a problem because the usual method of observing temperature is the ordinary thermometer, a device that can he observed by only one person a t a time. This difficulty has been met by calling upon a student from the class to read the thermometer, or avoided by omitting such demonstrations from the program. A sensitive temperature-indicating device that has a reasonably rapid response and would be visible to a large group would greatly increase the scope of phenomena that can be demonstrated to classes. To meet this requirement it was decided to build a thermometer that would indicate readings on a scale in front of a four-foot fluorescent lamp. Thermocouples, resistance thermometers, and thermistors are all available as sensing elements used in conjunction with suitable potentiometers or bridge circuits for translatiug electromotive force or resistauce into temperature readings. The thermocouple was selected for the present application because of its ease of construction, and economy. Since its output is measured with a potentiometer, the same circuit may be used to determine the electromotive force of electrochemical cells. One basic instrument can be used as hoth thermometer and voltmeter simultaneously. Information on the theory and construction of thermocouples is available from many sources. A useful
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LECTURE
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figure 2.
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Thermometer with Scale and Front ~~~~~~d
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SEPTEMBER, 1955
This makes it possible to place 0, 39,000 (R3), 399,000 (R3 R4), or 799,000 (R3 R4 R5) ohms in series with the 1000-ohm Helipot R2, and fixes the voltage drop across R2 a t 2, 0.05, 0.005, 0.0025 volts, respectively. (The fifth position on the switch S3 is not used on this model.) The selected value then represents full scale deflection of the indicator. By varying the position of the moving contact of the Helipot R2, any desired fraction of this voltage is fed into the amplifier in opposition to that furnished by the thermocouple Te. The t,wo, double-pole, double-throw reversing switches S4 and S5 make it possible to establish the zero point at the top or bottom of the scale. The converter amplifier is a Brown "Electronik" continuous-balance unit and the servo-motor designed to be used with it was obtained from the same s ~ u r c e . ~ When the potential difference between terminals Tx and Tz differs by more than a fraction of a microvolt, the output of the amplifier drives the servo-motor in a direction depending upon whether the thermocouple or the potentiometer is furnishing the higher potential. Connections from these terminals lead through the reversing switch S5 to one side of the thermocouple and through the resistor system t o the battery. The other sideof thethermocoupleconoectsdirectly to the battery. The shaft of the servo-motor is connected by gears to the rotor of the Helipot and to a sprocket that drives a film between the lamp and the scale. Thus, the sysFigieum 3. &hemstic S d i o n Showing S o m e D&ails of tem is self-balancing and indicates the balance point on Mech.nic.1 C o ~ t r v c t i ~ n the scale which may be calibrated in any appropriate units. Theedge of the black area marks the point on the scale corresponding to the value being measured. The film CONSTRUCTION forms a continuous loop that passes over spools a t the Five scales have been made for this instrument. top and a spool and sprocket a t the bottom of the scale These are +20 to - 13OoC.for measuring the tempera- housing. The scale inscriptions are painted on the back in. X 6'/~in. X 4 ft.,which ture of "dry ice," the freezing point of mercury, etc., side of a Plexiglas sheet 0 to -5°C. for freezing-point-lowering measurements, has been scoured with wet sandpaper to make i t trans0 to 130°C., 0 to 1200°C., and 0 to 2 volts. This last lucent. The scale slides in slots a t the front of the scale is used for the measurement of the voltage of supporting column, and additional slots are provided electrochemical cells. With the +20 to -100°C. range a copperconstantan thermocouple is used. For the 0 to -5'C.range a ten-junction chromel-constantan couple was constructed, since this metal combination gives the largest response per degree CONYeRTLR of temperature change. For the higher tem- , l o ; c . ~ L peratures a chromel-alumel couple is used. T. TL Many other combinations are, of course, pos$7 sible. To avoid the necessity for fixed junction I compensation we use an ice bath for the reference junction with all but the +20 to - 100°C. scale. For the latter, water a t 20°C. is used as a reference. The indicator is a strip of 35-mm. photographic film, half of which has been exposed and developed, the other half remaining clear.
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' Purchased from the Minneapolis-Honeywell Regulator Co., continuous balance unit part No. 353170-18, motor part No. 76750, 27 r. p. m.
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Elstrical Circuit.
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at the hack of the housing for the storage of the scales not, in use. The potentiometer and amplifier are housed in a conventional metal radio cabinet. The scale box frame is constructed of 1- X 2-in. aluminum channel. Three '/,-in. aluminum plates are placed a t the top, bottom, and four inches up from the bottom. The fluorescent lamp and reflector occupy the space between the upper t.wo plates. The spools mounted on the two upper plates guide the film which is driven by a sprocket from a discarded 35-mm. projector. The entire column is euclosed in sheet aluminum. An access door has been cut in one side to facilitate installation of the film. The servo-motor, gears, Helipot, and film-drive sprocket are housed in the bottom of the scale box. The motor drive operates a t 27 r. p. m. with no load. The Helipot requires ten revolutions to traverse the entire range and the film-drive sprocket is 3.14 inches in circumference. A 20-tooth gear is mounted on the motor shaft and this drives a 40-tooth gear on the Helipot and a 28-tooth gear on the film spool. This arrangement gives 42 inches of film travel for ten revolutions of the Helipot contact, the useful length of the scale.
JOURNAL OF CHEMICAL EDUCATION
arated rapidly by pouring through a large funnel fitted with a screen. The solution is weighed, and we have the following data for an example using 30 g. of urea as solute. Weight of beaker plus solution Weight of beaker plus solute Weixht of water Weiiht of solute Molecular w e i ~ h tof solute Freezing point of solution 30 g. solute X 1000 g. wster 560 g. water
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830 g. 270 p. 560 e. 30 g. 60 -1.63
53.5 g. solute/lMM g.. water
53.5 g. aolute = 0.89 mold 60 (mol. wt.)
E3 = 1.8 molal freezing-point oonetltnt
0.89
Determinations of this sort neglect many of the refinements required for precision measurements, but these are sacrificed in order to obtain reasonable s ~ e e d for a lecture demonstration. Determinations made in this manner can usually be completed in less than five minutes and give values ranging from 1.7 to 1.9. The experiment may then be repeated with different quanOPERATION tities of the same solute or with other solutes. We 111 operation the battery switch S1 and voltmeter usually determine the constant using one solute and 92 are closed; the voltage is adjusted to two volts with then use the constant thus obtained to calculate the RI. The appropriate source, thermocouple or electrn- molecular weight of a second "unknown" solute. Temperature Measurements. The temperature of vhemical cell, is plugged in with banana jacks and the range switch S3 set a t the appropriate position. The any object may be measured if an appropriate thermoscale is illuminated by closing the lamp switch S6 and couple and range setting are used. As an example the the amplifier is turned on with 87. If zero is to he at 20 to -130' scale is put in place, the range switch set the top of the scale switches, S4 and S5 are placed in a t position number 3, and a single-junction copperthe "up" position or in the "down" position for zero a t constantan thermocouple is connected to the instruthe bottom of the scale. The zero point may he ad- ment. With the reference junction held a t 20°C. and justed by raising or lowering the scale by means of the the other junction held between pieces of dry ice the knurled knob a t the hottom. The instrument is now thermometer indicates a temperature of about -78°C. With the single-junction chromel-alumel thermoready for use. couple, the range switch set a t position 2, and using the USES 0 to 1200°C. scale, the temperature differences in varFreezing-point Lowering. Freezing-point lowering ious portions of the Runsen-burner flame are readily experiments have often been included in laboratory demonstrated. Voltage Measurements. The 0 to 2 volt scale is put programs, hut much less frequently as lecture demonstrations. The technique which is described below is in place. Leads with hattery clips are connected in essentially that observed by the author as performed place of the thermocouple and the range switch set a t by the late Professor Rumold a t Kent State University, position 1. When the clips are fastened to the termiKent, Ohio. As carried out by him, the temperatures nals of a Daniel cell the meter will indicate 1.1 volts. were recorded with the usual laboratory thermometer. Other cells may he used to demonstrate the e. m. f . The development of the lecture-table thermometer series. Other Uses. It is anticipated that this instrument makes this experiment much more feasible as a lecture demonstration. The thermometer is equipped with a can he adapted to pH and gas-pressure measurements. 0 to -5' scale and the appropriate setting is made with ACKNOWLEDGMENT the range switch. The scale is arranged so that 0 is at Acknowledgment is gratefully made to Dr. R. J. the top. With both junctions in ice-water mixtures Jeffries of the electrical engineering department of the instrument reads O°C. A known weight of solute is dissolved in water, the Michigan State College for suggestions which made the solution poured over crushed ice, and the equilibrium device practical, and to Dr. H. R. Thompson of this temperature measured. The ice and solution are sep- laboratory for advice freely given.
