Barometers are instruments used to measure the pressure of the atmosphere. A barometer is used by meteorologists to forecast short-term changes in the weather. If the atmospheric pressure falls, storms and rain can be expected. There are two types of barometers that work differently to measure the atmospheric pressure.
Normally barometers require very little attention, but bad repair work can result in poor performance and in extreme cases the devaluation of a valuable item. This mainly affects the case. The barometer should be kept away from extremes of humidity and temperature, and as dust free as possible.
Nearly any barometer can be safely moved for a short distance without special preparation, but care must be taken to prevent (a) air from entering the mercury tube, (b) mercury from being lost, or (c) the rather fragile glass tube from being broken. Keep it tilted at all times; this will prevent the mercury from surging up and down in the tube and either drawing in a bubble of air or breaking the end out of the tube. If the barometer is to be stored for a short time, prop it as nearly vertical as possible in a safe corner. Return it to its home on the wall in the same way. Smooth and easy does it. Make no sudden movements.
A move to another household is a more serious matter. Probably the best advice is to take the barometer in the car yourself and don’t let the movers touch it. If there’s no way to avoid giving the barometer to the movers, prepare it as described below and then supervise while the packers pad a wardrobe box with pillows and pack the barometer in the proper position inside. Surround it with pillows so it can’t shift during handling.
Aneroid Barometers are made without fluid. It consists of a small, flexible metal box called an aneroid capsule, which is made from an alloy of beryllium and copper. The metal box is tightly sealed so that changes in atmospheric pressure outside the box cause an expansion and contraction of levers and springs inside the box. They can usually be adjusted by inserting a screwdriver into a hole in the back of the barometer. Care should be taken to ensure that the screwdriver engages with the slot in a small screw buried within the hole. It will usually only need a small fraction of a turn to change the setting of the pointer.
Mercury Barometer is a weather instrument was invented by Torricelli in 1643. A mercury barometer consists of a glass column marked off in inches. The top end of this glass tube is closed, and the other end rests in a small cup of mercury, called a cistern. A column of mercury resides inside the upright glass tube. Mercury barometers are often used today in physics classes. A Mercury barometer will show a normal reading of mercury at about 29 inches, which is the average barometric pressure at a sea level. At the time of a storm, there is less atmospheric pressure on the cistern. The barometer, in turn, shows the fall in mercury levels. As the storm passes, the low atmospheric pressure is replaced with a high-pressure system, which raises the level of mercury in the mercury column.
Mercury barometers are more difficult to regulate as it usually means the tube has to be adjusted in the case. In the case of scientific instruments and marine barometers containing mercury, this should only be done by an expert.
Mercury barometers need to be plugged unless they are of the enclosed safety (portable) type. To plug the average domestic wheel barometer which has a tube open to the atmosphere you need to carefully take the barometer off the wall and rest it vertically. Then carefully remove the glass float within the glass tube containing mercury and lean the barometer slowly over by about 45 to 50 degrees. You will see that mercury in the tube will go to the top of the tube. Don’t do this too quickly otherwise you might break the glass tube. Then carefully fit the retaining plug into the open part of the U tube containing the mercury, making sure that the point with the cotton tip is placed firmly into the small opening at the bottom of the wide part of the tube and that the cork sits within the open top of the tube. The glass can be extremely brittle, especially if it’s old, so take great care when doing this not to push too hard. Check that the plug is correctly fitted by bringing the barometer upright. The mercury should stay in the top of the tube. If there is no plug then place a small piece of cotton wool carefully into the top of the tube and keep the barometer at an angle of 45 to 50 degrees. It is possible to carry a barometer around in a car strapped in well to the front passenger seat with the seatbelt, provided this angle is maintained, though it is better if a passenger holds it.
Stick or straight-tube barometers
Stick or straight-tube barometers are made so that you read the mercury directly from the top of the visible glass tube. They will generally have either of two types of mercury reservoir (cistern) at the bottom of the glass tube: a cylindrical wood cistern with a closing screw, or a glass bulb. To determine which you have, you’ll probably have to remove the round wood cistern cover from the bottom of the case. It will either pull straight off forward, be held on by two small brass pins in the wood, or will have to be removed by turning the case around and taking two screws out of the rear. Keep the barometer upright while you do this.
Wood cistern with closing screw
The cylindrical cistern, or reservoir of mercury, is sealed to the bottom end of the glass tube, and also has a leather diaphragm in its base. The closing screw, more properly called the portable screw, is a threaded brass rod that protrudes from the bottom of the barometer. When the screw is turned upward, it compresses the diaphragm and expels some of the air from the cistern.
Take the barometer gently off the wall and tilt it slowly back, to about 45 degrees. The mercury will rise in the glass tube and fill the empty space at the top. With the instrument still tilted, turn the closing screw clockwise until you have pushed the leather diaphragm upward about one-quarter inch.
Don’t force the screw, or you may tear the leather out of the cistern. The barometer should now be gently laid flat and kept horizontal until you are ready to hang it again. When you get the barometer to its new home, hang it on the wall and then open the closing screw as far as it will go.
Glass bulb cistern
Stick barometers without a wood cistern and closing screw have a glass tube that is bent in a short “J” shape at the bottom. The short leg of the “J”, which is behind the removable wooden cistern cover on the bottom of the case, is topped with a glass bulb about half full of mercury, with an open spout. With the barometer in your hands, held vertical, and the cistern cover off, tilt the barometer back slowly and let the top of the tube filled with mercury. Then cork the open spout so the mercury can’t splash out. A size 00 or 000 cork will do and can be found at hobby shops or hardware stores. If you can’t find a suitable cork, plug the open end of the tube with a cotton ball and tape it over very thoroughly. Carry the barometer in the car at a 45 degree tilt only; never lay it flat. It will travel safely if you prop it in the back seat with pillows, and cushion the bottom end against sharp bumps.
Banjo (Dial or Wheel) barometers
Banjo (Dial or Wheel) barometers all have essentially the same form of mercury system. To prepare one for a long-distance move, you’ll need two sizes 00 or 000 corks, which can be found at hobby and hardware stores, and a little masking tape. Take the barometer gently off the wall, turn its back toward you, and set it on an upholstered chair or couch, tilted 45 degrees off of vertical.
Open the back door. The wood door may bind a bit, so work it open gently and resist the urge to pry it open with a screwdriver. Inside the back is a brass pulley with a string hanging from each side of it. Be very careful to keep these strings from unwinding from the pulley. Cut a one-inch length of masking tape and lay it across the strings, just below the pulley, taping both strings down flat against the inside of the case. Cork the open end of both tubes, with their strings and weights still inside. Keep the barometer tilted at 45 degrees at all times until you hang it on the wall again. Carry it in the car cushioned, plastic bag, and supported.
When you again hang the instrument, just reverse the steps above. First take out the corks, then be careful when you pull off the masking tape below the pulley, to prevent pulling the strings out of their grooves. Tilt the barometer back to vertical and hang it.
Wheel (banjo) barometers must be corked by a specialist before being moved.
THINGS TO REMEMBER
- Less fragile surfaces can be wiped with a soft lint-free cloth.
- Avoid metal polishes which may seep into the movement or destroy a valuable patina, like the patina on a brass carriage clock.
- For stubborn marks on a glass face use cotton wool damped in a mild detergent solution or methylated spirits. Rinse with damp cotton wool and buff gently with a chamois leather.
- Major cleaning of parts especially if dismantling is involved should be left to a clock specialist.
- Even oiling the mechanism of a valuable clock can be risky if you don’t know where to apply it, and too much or too thick an oil can attract abrasive grime which can affect the mechanism.
- Remove any gold or silver jewelry before handling a barometer. Mercury amalgamates with precious metals and makes permanent, ugly grey stains on them.
- Transport any barometer in a plastic bag. If you should have a mercury spill, it will be contained safely.
- Don’t let a barometer stay in a closed, hot car too long. The mercury will expand and pop the cork out or squeeze through the leather cistern bottom.
- If you have to move an open barometer around, move in slow motion. If the mercury surges to the top of the glass tube, the tube may be broken.
- If there are any small, loose parts, such as the set knob below the dial on a banjo barometer, put the parts in a baggie and rubber band them to the case.
- Don’t forget to ensure your barometer along with your other antiques. Most household contents policies offer a fine arts rider at a small extra cost but may exclude antiques under ordinary coverage.
- As you may be aware, exposure to large accumulations of mercury vapor over a long period of time can be harmful.
- The very small amount of mercury in your barometer, usually less than four ounces in a fully-filled banjo and less than six ounces in most sticks, which is exposed to the air in a room with average ventilation carries minimal risk.
- Do treat even a small spill of mercury with respect, though, and dispose of loose mercury in a tightly sealed container at a disposal site that takes used batteries. Such small quantities of mercury have no commercial value.
- Remember that the barometric pressure seldom changes more than an inch or so at any time. Dramatic fluctuations only occur with the very severe weather, so don’t be alarmed if your barometer doesn’t seem to move for weeks on end, especially during very hot summer weather.
- Don’t try to take your barometer with you on an airplane, no matter what the seller tells you. All airlines have very stringent regulations about hazardous materials, and mercury is high on their list. If you are caught trying to sneak a barometer aboard a flight, you will at the very least be detained, and at the worst could lose the barometer and/or be heavily fined. Plan ahead and ship the instrument, or better yet buy it from a local specialist.