Some Properties of Matter. Грекова О.А. - 11 стр.

                                                11
        A common form of thermometer is the mercury thermometer, consisting of a glass
bulb attached to a stem with a fine bore. The bulb and a short part of the stem are filled with
mercury. When the thermometer becomes warmer the mercury expands. Of course the glass
expands too, but not as much as the mercury, so the mercury moves up the scale as the
temperature rises.
        It is necessary to have standard temperatures. The temperature at which water freezes is
a fixed temperature and is used to graduate the thermometer. The temperature at which water
boils is another temperature which is fixed if the atmospheric pressure is fixed. If, however, the
atmospheric pressure is above normal the boiling-point is a little higher: this is because the
water has to be raised to a higher temperature for the steam to push back the greater pressure of
the atmosphere. Similarly, if the pressure is low, the boiling-point is low. Up high mountains
the atmospheric pressure is low, because there is a lesser thickness of air pressing down, and
water boils at a much lower temperature than usual, so that it is difficult to make good tea. The
temperature, then, at which water boils at a particular fixed pressure is taken as another
standard temperature.

                                       Comprehension Check:
                                Correct the wrong information (Part I)
      1)   All solids and liquids expand as their temperature rises and there are no exceptions
           from this rule.
      2)   The invar thermometer is widely used all over the world.
      3)   If the pressure is low the boiling-point is high.
      4)   Up high mountains water boils at much higher temperature than usual, so that we
           can make good tea.

                                                 II
       There are different temperature scales in use. The one used in all scientific work is the
centigrade scale, on which the temperature of melting ice is called 0 0 C, and the temperature of
water boiling at standard pressure is called 100 o C.
       The space on the stem between 0 o C, and 100 oC, is divided into a hundred equal
divisions. The scale is also called the Celsius scale, after the Swedish astronomer Celsius, who
first put it forward, so that C. can stand for this name as well as for centigrade. On the
Fahrenheit scale, the freezing-point is called 32o F, and the boiling-point 212o F., so that 180
Fahrenheit degrees correspond to 100 centigrade degrees. This means that 0 o F. is below the
freezing-point and equal to – 17.8 o C.
       There are other forms of thermometer depending on expansion. If two pieces of metal
which expand unequally are fastened side by side and the strip is heated, then the fact that one
side becomes longer than the other causes the strip to bend. This bimetallic thermometer is
sometimes used. More important, however, is the gas thermometer.
       The volume of a gas depends not only upon the pressure, but upon the temperature. If
the temperature rises and the pressure is kept constant, the gas expands: if the volume is to be
kept constant, the pressure must be increased. There are gas thermometers based upon both
principles : in the constant-pressure gas thermometer the change of volume measures the
temperature, in the constant-volume gas thermometer it is the change of pressure. The gas
thermometer is not used for ordinary temperature measurements, but it is the standard for
accurate temperature. The gas thermometer finally checks standard mercury and other
thermometers.