546 Prof. H. L. Callendar on Electrical 



or that the heating effect of the current must be increased 

 more than 40 per cent. The most important error to avoid 

 is to make R small compared with G or S. The sensitive- 

 ness may in this way be very greatly reduced, and the heating 

 effect of the current increased five or ten times. Thus, a 

 pyrometer of low resistance designed for work at high 

 temperatures (where the resistance is high and an open scale 

 is not required) would not be a suitable instrument to 

 employ for recording small variations of temperature on an 

 open scale at low temperatures, unless G and S were made 

 inconveniently small, and the pyrometer itself specially wound 

 with extra thick wire to give a large cooling surface. 



Construction of Thermometers. 



6. The construction of the thermometers depends on the 

 situation in which they are to be used. I have made three 

 principal types: — (1) For insertion in the mouth; (2) for 

 the rectum ; (3) for the surface of the skin or for the axilla 

 (armpit). 



(1) The mouth is not a suitable position for records of 

 long duration, but it is of interest to be able to insert an 

 electrical thermometer in this way in order to investigate 

 the conditions of lag, which cannot be observed so accurately 

 with a mercury clinical thermometer. It is most essential 

 that the thermometer should be of small thermal capacity, 

 in order to minimise the effect of inserting a cold thermo- 

 meter, since the tissues of the body are not very good con- 

 ductors of heat. In any case, if the mouth has not been 

 kept shut for some time previous to the insertion of the 

 thermometer, there will be a large apparent lag due to the 

 recovery of temperature of the mouth itself. A so-called 

 half -minute clinical thermometer may take upwards of five 

 minutes to get within a half a degree of the true temperature 

 under these conditions. I have tested several recording 

 thermometers from this point of view, and the results are of 

 some interest. 



Flat Glass Bulb Thermometer. — The ordinary type of 

 platinum thermometer has the wire wound on a mica cross 

 with compensated leads insulated by mica disks. Thermo- 

 meter and leads are slipped into a containing tube about 

 10 mm. diam., which is removable, and can easily be replaced 

 if damaged or broken, and which permits withdrawal of the 

 thermometer for adjustment or repair, if required. This 

 type is most convenient and suitable for ordinary laboratory 

 work with sensitive galvanometers ; but is insufficiently 



