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HE practical usefulness of a thermoelement will often depend •*• on whether its readings can readily be interpreted in terms of the standard scale of temperature, that is, on the ease and accuracy with which it can be calibrated. The present work was undertaken for two reasons, first, to obtain a calibration curve for a particular pair of elements which had been in use at the Geophysical Laboratory, and second, to obtain data relating to the calibration of copper-constantan thermoelements in general. The two elements in question were compared with two platinum resistance thermometers, which had been very carefully calibrated in terms of the international hydrogen scale. 1 The comparison was made at the Bureau of Standards, the thermoelements and accessory apparatus being brought over from the neighboring Geophysical Laboratory. The thermoelements were the A and B briefly described in the preceding article. They were read according to the method already published, 2 using two leakage shields and the simple potentiometer there described. The resistance thermometers were used substantially according to the method followed in the original calibration. A water-bath, which is ordinarily used for the testing of clinical thermometers, 3 was utilized for the comparison. During the progress of the work the bridge, the potentiometer and the standard cell were calibrated. The data for the resistance thermometer were most conveniently worked up in sets of four. The corresponding thermoelement readings were simply averaged. Each observation here given therefore represents the mean of four readings. The resistance thermometers were read to o°.oooi and the thermoelements to o°.ooo3, the P-337-No. 2.] CALIBRATION OF THERMOELEMENTS.