Part 2

Components of Barometer

Cistern Components

Adjust Mercury Level  Rotate adjustment knob at the bottom of barometer until the mercury in cistern just touches the ivory index.  Shine flashlight on white panel to see index.

Read thermometer then adjust vernier  Rotate vernier knob until bottom of vernier just touches top of meniscus. There should be no light between vernier and top of miniscus (some will show on the sides).

Reading Pressure

Calculate the Pressure  The uncorrected station pressure is the sum of the two readings:  Add the additional corrections: l Index correction. l Temperature l Gravity

Reset Cistern  Lower the mercury in the cistern until the index no longer touches the mercury.

Fixed Cistern Barometers  Kew Barometer l Uses large cistern so change in height of mercury in cistern is small. l Uses modified scale to compensate for no zero adjustment.

No Cistern Barometers  Siphon Barometer or Manometer

 The forces acting on each side of the siphon are:  Where,  Hg = density of mercury  a = density of air l g = acceleration of gravity l h AC = height of mercury from A to C l h AB = height of mercury from A to B l h BT = height from B to top of atmosphere l Area = cross-sectional area of tube

 Since pressure on the open side equals the pressure on the closed side, then solving for P a gives:

Weather Glass, Thunder Glass, Storm glass, Water Barometer

Additional Errors of Mercury Barometers  Mounting Errors l Non-vertical mounting results in error of zero point  Movement of barometer l Ships: Vibrations and waves cause inertial forces on mercury which act as additional gravitational forces affecting the length of the mercury.

2. Elastic Barometers  A direct measuring instrument in which the elastic deformation of a solid is used as an indication of atmospheric pressure. l Bourdon Barometer l Aneroid Barometer

Bourdon Barometer  Closed, curved, elliptical tube which changes curvature due to changes in atmospheric pressure  Not often used in Meteorological work l Has slow time response  New designs developed for oceanographic work and high pressure systems

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