THE THERMOMETER 437 



mercury that mijflit l)c drivcii over by accidental heating beyond 46° C. 

 Usually the tlienuonieter is made self-registering by having a small detached 

 thread of mercurv which is pushe<l up by the expanding fluid and remains 

 at the highest temperature reached. 



Where a continuous record lias 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 resisla))ce 

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

 ture. The alteration in conductivity may be measured by a Wheatstone's 

 bridge or may be recorded photographically as in the study of the electrical 

 changes in tissues (q.v.). 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. A half-minute 

 thermometer should remain at least 3 minutes 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. (Average value = 36-87° C.) 



(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. (Average value = 37-2° C.) 



(h) 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. (Average value = 36-9° C.) 



(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 clinieal sign, yet it does vary regularly by about 

 ± 0-5° C. during the 24 hour day. From the chart (Fig. 98) it 



