338 REGULATION OF TEMPERATURE 



Where a continuous record has to be made, or where great accuracy is 

 desired, one of the electrical methods is employed. 



(6) This method depends on the alteration of the electrical resistance 

 of a platinum (or other metal) wire caused by a slight increase in its tem- 

 perature. The alteration in conductivity may be measured by a Wheat- 

 stone's bridge or may be recorded photographically as in the study of the 

 electrical changes in tissues (p. 297). This is by far the more sensitive 

 of the two electrical methods, but great care must be exercised in its use. 

 It has, however, some disadvantages. For example, the current flowing 

 through the thermometer tends to heat the fine coil of wire. The heating 

 is proportional to the square of the current, and to the resistance of the 

 wire. 



(c) The thermopile is based on the principle that, if the junction of two 

 dissimilar metals (e.g. constantan and iron) be warmed, a difference of 

 potential between the two will be produced. In order that small fluctua- 

 tions of temperature may be measured, a second thermopile is arranged 

 in the circuit in series with but opposed to the first. This second thermo- 

 electric junction is kept at a certain known temperature differing but 

 little from the temperature to be measured. By this means the bulk of 

 the electromotive force resulting from thermopile 1 will, be neutralised by 

 the E.M.F. produced by thermopile 2, and thus the E.M.F. produced by a 

 small change of temperature becomes a relatively large proportion of the 

 net E.M.F. The current changes are read from a potentiometer. 



Location of Thermometer. 



(a) Natural Cavities. 



(i) Mouth. The cavity underneath the tongue is generally chosen by 

 the physician as suitable for the taking of temperature. The thermometer 

 should remain at least 7 mins. in situ. If the mouth is kept open, low 

 readings will be obtained due to the inrush of cold air and the vaporisation 

 of moisture. 



(ii) Rectum. Practically all physiologists are agreed that the most 

 favourable position for the thermometer for the indication of the true 

 temperature of the interior of the body is the rectum (or vagina in females). 

 The instrument should be inserted sufficiently deeply (7 cm.), to make 

 sure of the maximum temperature, and, in the case of the rectum, it 

 should not be imbedded in faecal matter. 



(b) Artificial Cavities. 



(iii) Axilla. In the private practice of a physician, temperatures are 

 generally taken from the armpit. Care has to be taken to free the skin from 

 moisture and sweat and to close the cavity on the thermometer for a time 

 sufficiently long to enable the surface of the skin to attain the temperature 

 of the interior of the body. 



(iv) Groin. In young children the thermometer is usually placed in 

 the fold of the groin. 



While the temperature of the body is so constant that it is 

 taken as a clinical sign, yet it does vary regularly during the 

 24 hour day. From the chart (Fig. 72), it will be seen that 

 between 3 and 4 in the morning body temperature is at its lowest 



