HEAT - TEMPERATURE AND SCALES

HEAT - TEMPERATURE AND SCALES
Thermometer:
*     It is a device used for measuring the temperature of different objects.
*     First thermometer was invented by Galileo in 1593 AD. In this thermometer air 
    was used as the thermometric substance,as the air rapidly expand on heating        and  contract on cooling.
1.    Clinical Thermometer : 
1)     It is used for measuring the temperature of the human body. 
2)    It is graduated in between 35 - 42⁰C (or) 95 - 108F
3)     In this, the bend (constriction) in the bore does not allow the mercury to fall back  in the bulb after its use, unless the thermometer is vigorously shaken. 
4)    The normal temperature of a human body is 98.4⁰F or 36.9⁰C.
2.    Maximum and minimum Thermometers :
    For recording the maximum and minimum temperatures reached on any particular day,  maximum and minimum thermometers are used. They are:
    i)  Rutherford’s maximum Thermometer. 
    ii) Rutherford’s minimum Thermometer. 
    iii)Six’s maximum and minimum Thermometer.
        The other Thermometers are
    i)  Laboratory thermometer     ii) Alcohol Thermometers,etc.  
 Reasons for using mercury in Thermometers: 
1)     Mercury needs very little heat to expand. Thus, it can easily record the temperature of a body without changing the temperature of the body. 
2)     The temperature at which mercury boils is 357⁰C. Thus, it can be used to record temperatures as high as 357⁰C. 
3)     The temperature at which mercury changes into solid state is - 39⁰C. 
4)     Mercury does not stick to the sides of the capillary tube. Thus, it helps in 5)     Mercury is opaque and shining. Thus, it can be easily seen as a fine thread in
    the capillary tube. 
Reasons for using Alcohol in Thermometers:-
1)     Freezing point of alcohol is less than - 100⁰C. 
2)     Alcohol expands more than mercury for the same rise (or) fall in temperature.
    Thus, alcohol thermometers can measure more accurately than mercury. 
3)     Alcohol can be brightly coloured and hence,  can easily be seen in the glass capillary tube. 
Standard points (or) fixed points:- The fixed temperatures marked on the stem of a  thermometer by the common consent of scientists. 
Lower standard point (or) lower fixed point : The melting point of pure ice at a pressure of 76 cm of mercury. 
Upper standard point (or) upper fixed point :The boiling point of pure water at a pressure of 76 cm of mercury.