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EXPERIMENT NO. 416

THERMO-COUPLES.

References: Stewart, Physics, Sect. 190; Kimball, College Physics, Sect. 662-664; Duff, College Physics, Sect. 333, 334; Spinney, Text-Book of Physics, Sect. 169.

One type of thermometer, which now finds many different uses, is the thermo-electric thermometer or thermo-couple. This experiment is designed to show qualitatively what happens as one junction is heated, and also to show how to work out a calibration which would permit the use of the thermo-couple as a thermometer.

In a circuit composed of two different metals, if one junction is kept at a constant temperature, while the other is heated, an e. m. f. is generated, and a current flows in one direction or the other, depending on the nature of the metals and which junction is the hotter. This current will rise to a maximum, then diminish to zero, and finally reverse as the temperature difference between the two junctions increases. If the e. m. f. or the current is plotted against the temperature difference, a parabola results, but with most metals, and temperatures not greatly exceeding 100° C., the graph is almost straight, being an are of the less curved portion of the ascending branch of the parabola.

(a) Three thermo-couples are provided, two are copperiron, the other is copper-advance (advance is the trade name for a copper-nickel alloy). One copper-iron couple is mounted so that one junction can be heated by means of a Bunsen burner while the other remains at room temperature. Connect the terminals of this couple directly to the galvanometer, heat one junction and note the deflections resulting. Record the general behavior of the instrument as the junction is slowly brought up to a red heat and then allowed to cool.

(b) One junction of each of the other couples is in a testtube of oil, which may be cooled in ice or kept at room temperature, the other junction is in a beaker of oil, which may be heated by a burner. Stirrers and thermometers are provided for equalizing and reading the temperatures. The beaker and test

tube may be mounted at opposite ends of the cross-arm of a T-shaped wooden frame.

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Connect up as in Fig. 15, heat the beaker slightly, stir thoroughly after removing the burner, read the two temperatures and take the galvanometer deflections given by the two couples in turn, using the switch, S, to throw first one and then the other into circuit with the galvanometer. It is usually necessary to put about 500 ohms resistance in the copper-advance circuit to keep the deflections on the scale at the higher temperatures, but none is needed with the copper-iron couple. Keep R the same throughout the experiment. Record the galvanometer number and the value of R used, for it is obvious that the calibration of any thermo-couple as a thermometer is valid only for the particular galvanometer and circuit used in the test. Raise the temperature of the hot junction up to a little above 100° C, in ten steps of approximately equal values, taking temperature readings and galvanometer deflections for both couples at each step.

If it is merely desired that the couples be calibrated as thermometers, a graph should now be plotted, using temperature differences between the junctions as abscissæ and galvanometer deflections as ordinates. Any temperature within the range of the test can be found by the use of this graph, when the corresponding galvanometer deflection is known. It must be kept clearly in mind, however, that what is indicated is the difference in temperature between the two junctions. Unless the cool junction is at the temperature of melting ice, this difference is not the actual temperature reading. The beaker

of oil may be removed and the couples used to measure the temperatures of a number of baths at different temperatures. The results should be compared with readings of a mercury thermometer placed in the same baths.

EXPERIMENT NO. 417

COMPARISON OF SMALL CAPACITANCES, USING OSCILLATORY CIRCUITS.

References: Stewart, Physics, Sect. 550, 552; Kimball, College Physics, Sect. 758, 759; Duff, College Physics, Sect. 380382; Spinney, Text-Book of Physics, Sect. 391-394.

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In Experiment No. 415 there was given a method for the comparison of capacitances by means of a ballistic galvanometer. This method serves very well for comparing telephone condensers and others the capacitances of which range from 0.1 up to 2 or 3 microfarads, but will be of no use with condensers of the sizes commonly employed in radio circuits. These condensers have capacitances of the order of magnitude of 0.0001 microfarad.

The present method makes use of simple oscillatory circuits, and, as it is a substitution method, does not make necessary any computations of the characteristics of the circuits. If actual determinations of unknown capacitances are to be made, a calibrated, variable condenser is essential. The manufacturers of radio apparatus offer for a few dollars variable condensers of capacitances ranging from 20 to 2000 micromicrofarads, together with the proper calibration curves or scales.

In the circuit shown in Fig. 16, the sending oscillatory circuit contains a variable condenser, C1, of about the same capaci

tance as those to be compared, an inductance, L1, consisting of 10 to 20 turns of insulated wire wound in a circle about six inches in diameter. A battery, switch, and high-tone buzzer, B, make up the remainder of the circuit. A telephone condenser may be connected in parallel with the buzzer magnet, if desired. If a high-tone buzzer is not available, a small spark-coil may be used, the secondary being left open. The buzzer or coil must be at some distance from the receiving circuit and should be enclosed in such a way as to muffle the sound of the vibrator.

The receiving circuit contains a coil, L, similar to L, and a double-throw switch, S, by which the calibrated condenser, C3, or the unknown, C2, can be connected in as desired. The areas of the two oscillating circuits should be approximately equal and the two condensers, C2 and C2, should be so placed that the area of the receiving loop remains essentially the same for either position of the switch, S. D is a crystal detector and T a telephone receiver. An ordinary watch-case receiver gives better results than very sensitive instruments.

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Lay the two coils, L, and L2, side by side on the table. Put C2 in circuit and adjust C1 until the best response is obtained in the receiver. It may be necessary to increase the distance between L, and L2 to determine the best setting of C1. Then replace C2 by C, and adjust C, until the maximum sound is heard in the receiver. When this adjustment is made as accurately as possible, the distance between L1 and L2 should be increased and C, again adjusted. L1 and L2 are moved farther and farther apart and C, adjusted until finally the sound fades out at all positions of C, except one. This known value of C, must give the capitance of C, as nothing in the circuit has been changed except that C has replaced C2. If C, is adjustable, the process may be repeated for other settings of C2 and its whole scale calibrated.

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PHOTOMETRY.

erences: Stewart, Physics, Sect. 561-562; Kimball, College Physics, Sect. 794, 797, 799; Duff, College Physics, Sect. 414-415; Spinney, Text-Book of Physics, Sect. 523-527, 529-531.

The comparison of the luminous intensities of two lamps is difficult if the lamps differ as to the colors of light emitted. h simple photometers only approximate results can be ined, the accuracy depending on how closely the observer can nate equal illuminations on two parts of a screen. Briefly, theory is as follows, no matter which one of several types of Eometers is used.

Suppose a screen to be placed so that it is equally illumied by the standard and one of the unknown lamps. Then if e the candle-power of the standard and I, that of the unknown p, r1 the distance of the screen from the standard and r2 that a the unknown, the illumination due to the standard is 2 and that due to the unknown is 12/r22. Since the screen djusted so that these two illuminations are equal, we have,

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With the ordinary photometer box one meter long, the ps should not be of high candle-power. The glare from high

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