TEMPERATURE. 



13 



10 



10 



20 



40 



50- 



60 



70 



80 



90- 



IOO 



no 



120- 



130- 



ISO 



120 



Maximum and Minimum Thermometers. It is often important 



for meteorological purposes to register the highest and lowest tempera- 

 tures which have been attained in any 

 period during the absence of the observer. 

 For maximum thermometers, a common 

 device is to have a short rod of iron in the 

 tube above the mercury ; the thermometer 

 is then placed with its stem horizontal. As 

 the mercury moves outwards along the 

 stem, the rod is pushed in front of it; but 

 when the mercury recedes, it leaves the rod 

 behind, thus indicating the farthest point 

 reached. The iron index may be brought 

 back into contact with the mercury by means 

 of a magnet. 



For minimum thermometers, a small glass 

 rod is put in the tube, which is horizontal. 

 The liquid (in this case usually alcohol) flows 

 past the glass rod in rising, but in falling it 

 pulls the rod back with it owing to capillary 

 adhesion. The glass index may be brought 

 again to the end of the liquid column by 

 inverting the thermometer. 



Six's TJiermometer. This instrument, 

 which is a maximum and minimum thermo- 

 meter in one, is now very commonly used. 

 The construction is shown in Fig. 6. It 

 consists essentially of a U tube with a bulb 

 at each extremity of the U. The bulb G 

 contains alcohol or other suitable liquid, 

 extending along the tube to a; ab is a 

 thread of mercury extending round the bend 

 to b ; above b the tube and part of the bulb 

 H to which it leads are filled with the same 

 liquid as that in G, and above the liquid is 

 a space V containing the vapour of the liquid, 

 which can be compressed or extended, and 

 serves as a sort of spring. 



In the tubes above a and b are two 

 small iron rods or indices with hairs 

 attached to them, the hair giving just 

 enough friction to keep the index in posi- 

 tion when the mercury column retreats and 

 leaves it. The indices may be brought into 

 contact with the mercury at a and b by means 

 of a small magnet applied outside the tubes. 



Suppose that at a given temperature FKJ. g. 



the indices U are in contact with the 



mercury on each side. If now the temperature rises, the liquid in 

 G expands, pushes down the mercury from a, and the mercury thread 

 ab moves round, b rises and the vapour space in the bulb above it is 



80 



7O 



GO 



50 



40 



10 



o 



10 



