DIFFERENTIAL THERMOMETERS. 



advantage over liquids in retaining its gaseous state at all tem- 

 peratures, and in the perfect uniformity of its dilatation and 



Fig. 8. 



Co' 



10 0' 12,0 



Fig. 



o 



contraction. It is also highly sensitive, indicating changes of 

 temperature with great promptitude. Since, however, it is not 

 visible, its expansion and contraction must be rendered 

 observable by expedients which interfere with and 

 render complicated its indications. 



The air thermometer of Drebbel, or according to some 

 of Sanctorius, is represented in fig. 9. A glass tube, 

 A B, open at one end, and having a large thin bulb 

 c at the other, is placed with its open end in a coloured 

 liquid, so that the air contained in the tube shall have 

 a less pressure than the atmosphere. A column of the 

 liquid will therefore be sustained in the tube A B, 

 the weight of which will represent the difference 

 between the pressure of the external air and the air 

 inclosed in the tube. 



If the bulb c be exposed to a varying temperature, the air 

 included in it will expand and contract, and will cause the column 

 of coloured liquid in the tube A B to rise and fall, 

 thereby indicating the changes of temperature. 



Another form of air thermometer is represented in 

 fig. 10. The air included fills half the capacity of the 

 bulb c, and its expansion and contraction cause the 

 coloured liquid to rise or fall in the tube A B. 



36. Of all forms of air thermometer, that which 

 has proved of greatest use in physical enquiries is the 

 c differential thermometer represented in figs. 11, 12. 

 This consists of two glass bulbs, A and B, connected 

 by a rectangular glass tube. In the horizontal part 

 of the tube a small quantity of coloured liquid 

 (sulphuric acid, for example) is placed. Atmospheric air is con- 

 tained in the bulbs and tube, separated into two parts by the 



159 



Fig. 10. 



