STANDARD THERMOMETER. 



in the scale, which may be continued by like divisions above 150 

 and below 50. 



31. The range of the scale of thermometers is determined by the 

 purpose to which they are to be applied. Thus, thermometers 

 intended to indicate the temperature of dwelling-houses need not 

 range above or below the extreme temperatures of the air, and 

 the scale does not usually extend much below the freezing point 

 nor above 100; and thus the sensitiveness of the instrument may 

 be increased, since a considerable length of the tube may represent 

 a limited range of the scale. 



32. Mercury possesses several thermal qualities which render 

 it a convenient fluid for common thermometers. It is highly 

 sensitive to change of temperature, dilating with promptitude by 

 the same increments of heat with great regularity and through 

 a considerable range of temperature. It will be shown hereafter 

 that a smaller quantity of heat produces in it a greater dilata- 

 tion than in most other liquids. It freezes at a very low and 

 boils at a very high temperature. At the temperatures which are 

 not near these extreme limits, it expands and contracts with 

 considerable uniformity. 



The freezing point of mercury being 40, or 40 below zero, 

 and its boiling point + 600, such a thermometer will have 

 correct indications through a very large range of temperature. 



33. It is sometimes needed, in the absence of an observer, to 

 ascertain the variations which may have taken place in a ther- 

 mometer. Instruments called self- registering thermometers have 

 been contrived, which partially serve this purpose by indicating, 

 not the variations of the mercurial column, but the limits of its 

 play within a given time. This is accomplished by floating 

 indices placed on the mercury within the tube, which are so 

 adapted that one is capable of being raised with the column, but 

 not depressed, and the other of being depressed, but not raised. 

 The consequence is, that one of these indices will remain at the 

 highest, and the other at the lowest point which the mercurial 

 column may have attained in the interval, and thus register the 

 highest point and lowest point of its range. 



One of the most common and useful forms of self-registering 

 thermometer is that of Rutherford, shown in fig. 7, which consists 

 of two tubes, attached in a horizontal position to a plate of glass, 

 being bent at right angles near the bulbs ; one, A, containing 

 mercury and the other, B, alcohol. In the tube of the former 

 there is a small piece of iron wire, which moves in it freely, being 

 pushed along by the mercury as it expands. When the tube is placed 

 in the vertical position the wire falls back upon the mercury. 



The other tube contains a small piece of coloured glass, having 



157 



