The Measurement of Intravenous Temperatures

The temperatures of various parts of the body, as determined with the ordinary clinical thermometer are, in general, different, and the differences are not always consistent. We cannot assume, therefore , that such measurements give a true index of the mean internal temperature of the body, which may perhaps best be identified with the temperature of the blood flowing in large blood vessels. Moreover the instrument itself is not free from objection. Its heat capacity is sufficiently great to lower appreciably for some time the temperature of tissue with which it comes in contact, and its response is too slow to follow fluctuations found with more sensitive apparatus. Although measurements made with a clinical thermometer are doubtless quite satisfactory for most purposes, it is possible that, for certain kinds of experimental work, fairly accurate determinations of blood temperatures may prove interesting. Apparatus which has been devised for this purpose by the author is described below. The temperatures are measured by a thermoelectric method, in which one makes use of the fact that the discontinuity of electric potential at a junction of two dissimilar metals depends only on the nature of the metals and the temperature of the junction. A closed circuit of two metals contains, of course, two opposed junctions. If the second of these junctions is kept at a known constant temperature, the resultant thermoelectric" force in the circuit is the difference between the potential discontinuities at the two junctions, and this is a function of the temperature of the first junction only. This thermoelectromotive force may be used ill either of two ways to measure temperature. It may be measured directly by means of 385