24 TEMPERATURE FLUCTUATIONS IN THE HUMAN BODY. 



balance exactly, as this requires some time, but after the balance has been 

 obtained within perhaps 0.1 C, the potentiometer setting and the deflection 

 are observed and noted. From time to time the current is checked as at the 

 beginning, also the temperature of the constant-temperature bath is occasion- 

 ally taken and the correction for extraneous electromotive force determined. 

 For the purpose of making this last test the test switch T is provided. The 

 connections are such that if this switch is closed, instead of one of the switches 

 1, 2, 3, 4, etc., the apparatus is in the condition usual for a temperature 

 measurement, except that the thermal -junction system is replaced by a direct 

 copper connection. The electromotive force in this circuit should then be zero, 

 and if the potentiometer is set at zero, a balance should be obtained. If this 

 is not the case, a galvanometer deflection will result, indicating the presence 

 of an extraneous electromotive force. This error may be determined either 

 by changing the potentiometer setting until a balance is obtained, or else 

 having a knowledge of the sensitiveness of the galvanometer by interpreting 

 it directly from the deflection. The latter method, being quicker, is perhaps 

 preferable; it also frequently saves reversing the storage cell and standard 

 cell, which would be necessary in balancing if the stray electromotive force 

 happens to oppose the main voltage of the thermal-junction system. 



CALIBRATION OF THERMAL JUNCTIONS. 



In calibrating the thermal junctions, a Dewar flask was used, similar to that 

 in the constant-temperature oven. This was filled with water at about 

 body-temperature and immersed in a considerable mass of warm water in a 

 pail, the temperature of the outside water being controlled by an electric-coil 

 heater. The Beckmann thermometer (40723), already described, and all 

 the junctions to be calibrated were placed side by side and bound together 

 with rubber bands in such a way as to bring the junctions at about the middle 

 of the bulb, but without pressure on the bulb itself. The thermometer and 

 junctions were then inserted in the calibrating flask, the surrounding water 

 being stirred by compressed air from the supply in the constant-temperature 

 oven. After the temperature in the calibrating flask had become constant the 

 Beckmann thermometer was read, the necessary potentiometer readings were 

 made for each thermal junction, and the reading of the Beckmann thermometer 

 in the constant-temperature oven was taken. Usually four complete series 

 of readings were made as quickly as possible; their average furnished data 

 which showed the relation between temperature difference and potentiometer 

 balance for this point. After this another temperature was used in the cali- 

 brating flask and similar data obtained. In this way the junctions were 

 calibrated throughout the range of temperature difference likely to occur in 

 an actual experiment. 1 



'These conditions are also suitable fortesting the constantan wireforinhomogeneity. The 

 most important part of the circuit, namely, that where the temperature gradient is usually 

 steepest and most variable, occurs near the junction for use on the body. This region may be 

 explored by keeping the junction at a fixed temperature in the bath, meantime immersing 

 the wires to varying depths. A number of tests made in this way showed no change in the 

 galvanometer deflection, indicating that the wire was sufficiently homogeneous for this work. 



