Recording Thermometers for Clinical Work. 547 



sensitive for insertion in the mouth. Moreover, since the 

 wire is surrounded by air, the heating effect of the current 

 may be excessive when such a thermometer is used with 

 recording instruments on an open scale, especially if its 

 resistance is low. 



A great improvement on this type, in point of quickness 

 •of action and diminution of the heating effect of the current, 

 is readily effected by winding the wire on a flat plate of mica 

 in place of a cross, and melting the lower part of the con- 

 taining tube (which is preferably of lead glass) down on to 

 the wire so as to form a flat bulb, as illustrated in fig. 2. 

 If the glass is thin, the sensitiveness may be increased 

 nearly five times, as compared with a thermometer of the 

 ordinary type, and the heating effect of the current reduced 

 in nearly the same proportion. When inserted in a water- 

 bath at 37° 0. this thermometer takes less than a minute in 

 arriving within a hundredth of a degree of the final tern- 

 perature ; but when inserted cold in the mouth, it may take 

 4 or 5 minutes to get within a tenth of a degree, because 

 the tissues of the mouth take time to recover from the 

 cooling effect of inserting the thermometer, as further 

 illustrated below. Where quickness of action is an essential 

 condition, as in some kinds of calorimetric work, the flat- 

 bulb thermometer is a great improvement on the ordinary 

 type with a round tube, but it shares with the mercury 

 thermometer the disadvantage that, if broken or damaged, it 

 cannot easily be repaired. 



It is essential that thermometers of this type should be 

 provided with compensated leads, otherwise there will be a 

 variable immersion error, and an apparent lag due to slow 

 conduction of heat along the leads. It is also most important 

 for accurate work to avoid screw terminals in the head of 

 the thermometer, and to provide each instrument with 

 flexible leads two or three metres long, permanently soldered 



Fi*. 2. 



Flat Glass Bulb Thermometer. 



to the thermometer and compensator leads, and securely 

 attached to the head of the thermometer, as indicated in fig. 2. 



2^2 



