1891.] On a Compensated Air Thermometer. 251 



of the connecting tubes is not too large compared with that of the 

 bulb. It is often a matter of great convenience to have the thermo- 

 metric bulb at some distance from the indicating apparatus, and not 

 rigidly connected to it. Since the connecting tubes are compensated, 

 they may be made of considerable length and of flexible material, 

 such as compo. or even rubber tubing, without much loss of 

 accuracy. 



For moderate ranges of temperature, an auxiliary bulb for measur- 

 ing the dilatation of the air and adjusting the pressures to equality 

 may be dispensed with. The sulphuric acid gauge itself may be 

 graduated to indicate the difference of temperature between the two 

 bulbs. In ordinary work, however, it would be inconvenient either 

 to keep the standard bulb always at the same temperature, or to take 

 its temperature and do an addition sum at each observation. The 

 simplest way of avoiding this is to adjust the volume of the sulphuric 

 acid in the pressure gauge so that its expansion may compensate for 

 the dilatation of the air in the standard pressure bulb. This com- 

 pensation may readily be made sufficiently accurate over the small 

 range of temperature of the air of a workshop or laboratory. 



When the instrument is thus compensated for changes of tempera- 

 ture in the standard bulb, one tube of the pressure gauge can be 

 graduated directly in degrees to indicate the temperature of the 

 thermometric bulb. The indications are then as easy to read as 

 those of a mercury thermometer. They are not affected by changes 

 of temperature in the surrounding air or by variations in the height 

 of the barometer, and they are independent of the length of stem 

 immersed. The range covered by a single instrument may be 100 C. 

 or more, and may be made to correspond to any part of the scale by 

 suitably adjusting the volumes of the air bulbs and tubes of the 

 pressure gauge. 



I have found such thermometers* exceedingly convenient and 

 satisfactory for rough work at temperatures beyond the range of 

 mercury thermometers. They can be made to read easily to the 

 tenth of a degree at 450 C., and if properly compensated their 

 indications are very reliable. Such a degree of accuracy is amply 

 sufficient for most purposes, and the absence of all necessity for cal- 

 culation or correction of the readings is a very great advantage. 



* Perhaps I ought to mention that this direct-reading form of instrument has 

 been patented, owing to its many commercial applications. It is made by Mr. J. J. 

 Hicks, of Hatton G-arden, E.G. 



