════════════╤═════════════════════════════════╤═════════════════════ Temperature │ Pyrometer 1. Iron-constantan. │ Pyrometer 2. of Hot │ (Series resistance in │ Platinum- Junction. │ galvanometer circuit.) │ iridioplatinum. ├───────────┬─────────┬───────────┼───────────┬───────── │Deflection.│ Cold │Difference.│Deflection.│ Cold │ │Junction.│ │ │Junction. ────────────┼───────────┼─────────┼───────────┼───────────┼───────── 100° C. │ 8·9 │ 15° C.│ 85° C. │ 5·5 │} 232 │ 21·8 │ 17 │ 215 │ 15·6 │}Constant 419 │ 40·6 │ 19 │ 400 │ 29·4 │} at 631 │ 63·8 │ 19 │ 612 │ 45·5 │} 25° C. 800 │ 83·0 │ 20 │ 780 │ 59·0 │} 1084 │ ... │ ... │ ... │ 82·0 │} ════════════╧═══════════╧═════════╧═══════════╧═══════════╧═════════ [Illustration: FIG. 15.—CALIBRATION CURVES FOR TWO THERMO-ELECTRIC PYROMETERS.] Fig. 15, A, is a calibration curve for thermocouple 1, connecting deflections with corresponding differences between the temperatures of the hot and cold junctions. In order to read from this curve the temperature of the hot end, the reading corresponding to the observed deflection is added to the existing temperature of the cold junction. Thus if a deflection of 56 divisions were obtained with the cold junction at 25°, the temperature of the hot junction would be (540 + 25) = 565° C. The advantage of this method of calibration is that it is unnecessary to take precautions to keep the cold junction at a steady temperature; and when a single cold junction is used, as in fig. 6, this plan should always be followed. It will be noted that this curve passes through zero, as no deflection represents no difference of temperature. Fig. 15, B, represents the calibration curve for pyrometer 2, and is such that direct readings may be obtained corresponding to any given deflection, for a cold junction temperature of 25°. This curve, therefore, cuts the axis of zero deflection at 25°, as no deflection corresponds to the condition when both hot and cold junctions are at 25°. This method of calibration may be used with advantage for couples of the type shown in fig. 4, where two cold junctions exist in the head, and the simple rule of adding the cold junction temperature does not apply. Many suggestions have been made for correcting for alterations in the temperature of the cold end of such a couple, but none are accurate, and it is necessary to keep this part at the temperature of standardization to secure correct readings. In both of the above calibrations the galvanometer used possessed a scale divided into 100 equal arbitrary divisions.