Antique Barometers Specialist
IS YOUR MERCURY STICK BAROMETER READING CORRECTLY?
The basic mercury barometer is responding simply changes in atmospheric pressure. Usually, the reading will change from day to day – exceptionally from hour to hour. If this is not happening, your barometer is sick and may need expert attention.
Consider its past history. Have you moved house, or redecorated the room where it normally lives? If so, was it taken off the wall and laid down without ensuring that its siphon was plugged to prevent air getting into the tube? If air gets into the vacuum at the top of the glass tube, it acts as a cushion and prevents the mercury from rising.
Is the mercury level always below the bottom of the scale? You may have spilled some of the mercury when the instrument was being moved.
Look at the mercury in the tube. Can you see small bubbles trapped between the mercury and the glass? Other bubbles may have reached the top of the tube, or may be causing the surface of the mercury to stick in the tube, giving a false reading. Can you see a granular yellow deposit on or in the mercury? This is a mercury oxide which also inhibits a smooth flow of mercury along the tube.
All these defects can be made good by taking your barometer to a reputable craftsman who will remove the old mercury, clean the interior of the glass tube, and refill it with new mercury. Or in the case of the mercury tube being beyond repair, make a new
IS YOUR BANJO BAROMETER PLAYING THE WRONG TUNE ?
Behind your banjo barometer is a tall glass tube with its lower end turned up. The mercury column occupies about 76 cm (30 inches) of the tube, reaching to about 12 cms from the top. Floating on the mercury in the open limb is a glass weight which hangs on a cord passing over a pulley wheel and counterbalanced by a similar weight. As the column of mercury moves along the glass tube in response to changing atmospheric pressure, the float rises and falls, its cord turning the pulley wheel. The axis of this wheel carries a pointer, visible from the front of the barometer, which rotates round a graduated dial to indicate the pressure. There may be a second pointer which can be turned by hand to mark the previous day’s reading.
If the barometer reading does not vary, begin with the basics. Is there mercury in the tube? Is the cord supporting the glass weight and its counterweight frayed or broken?
If there is mercury and the float mechanism is undamaged, consider whether the barometer was moved without removing the glass weight and plugging the open tube. Has air entered the vacuum at the top of the tube? Is there a corrosion deposit on the open surface? Is there corrosion on the wheel and axis, or has the pointer become loose on the axis?
These defects can be remedied by cleaning the tube and refilling it with new mercury and mending the moving parts driving the pointer as necessary. Or in the case of the mercury tube being beyond repair, make a new replacement tube.
IS YOUR BAROMETER LOOKING DISTRESSED?
When you see a barometer advertised for sale ‘in a distressed condition’, you may expect to see peeling veneer, even chunks of beading lost. It may be missing sections of mother-of-pearl decoration, its brass top ornaments, or the knob moving its pointer.
How is the graduated scale? If it has a brass scale, is the silver plating corroded or worn? If the scale is ivory or bone, has it spilt or become discoloured?
Most injuries to the frame can be repaired and scales can be resilvered.
Page updated 19th February 2010