About Barometers A STROLL AROUND MY HEAD  Guide to Antique Barometers There are two main types of Barometer, the Mercury and the Aneroid. The Mercury Barometer was invented by Torricelli in 1643 during his investigations into pumping water out of mines. The Aneroid Barometer was invented by M. Vidi in 1844. Mercury Barometers A Wheel Barometer A Stick Barometer The mercury barometer comes in two main forms, the Wheel (or Banjo) and the Stick. Both measure the height of a column of mercury that is supported by air pressure: as the air pressure drops so the height of the column falls and as the air pressure rises so does the column height. In both forms, the mercury is enclosed in a vertical tube, at the top end of which is a vacuum, while the other end is open to the atmosphere. In the stick barometer, the open end is immersed in a bath of mercury, while in the wheel, the open end of the tube is bent up to form its own 'reservoir'. In many of the former type, the mercury level in the reservoir is adjusted by a screw to a set mark before reading the top of the mercury column against a scale. In the latter, the changing level in the 'reservoir' is used as a measure of the height of the column. This is done by having a glass float resting on the surface of the mercury. This float is hung on a thread which passes over a pulley and is kept taut by a counter-balance glass weight. In this way, the linear movement of the float is converted into rotary movement to drive a pointer round a scale, calibrated in inches or centimetres (the height of the column of mercury). Care of Mercury Barometers No mercury barometer should be moved suddenly, without taking proper precautions, for fear of introducing air bubbles into the mercury column, or, with wheel barometers, for fear of the mercury overflowing from the open end of the tube. Mercury is a poisonous metal which can be absorbed through the skin (see notes on transportation of mercury barometers). Air bubbles in the column upset the calibration of the instrument. Transportation of Mercury Barometers Stick barometers should have their adjusting screw turned up until there is no vacuum gap at the top of the tube, but take care, as sometimes they have been insufficiently filled, so if there is an increase in resistance to turning the screw, stop, otherwise there is the danger of the screw puncturing the leather membrane that separates it from the mercury. If for some reason the vacuum gap cannot be closed then tilting the tube by 45 degrees will accomplish it. The barometer can then be safely transported, preferably upside down! This only applies to stick barometers! Wheel barometers should have a piece of masking tape applied over the pulley and the threads coming from it, to stop them coming off the pulley. The glass float may then be carefully drawn from the tube and allowed to hang beside it. Tilt the barometer slowly from the vertical until there is no vacuum gap at the top of the tube. The open end of the tube may then be plugged with a special plug shown below. The cotton wool is forced into the narrow tube at the bottom of the reservoir while the cork seals the top so that the mercury is trapped. If one of these is not available, a plug can be improvised by pushing a plug of cotton wool with a thread tied around it (so it can be retrieved) down to the bottom of the reservoir with a thin wood pencil (don't use metal as mercury will amalgamate with many metals). Then push a tightly fitting cork into the open end. When these precautions have been taken, the barometers can be safely transported at an angle of about 45 degrees to the horizontal. In a car, it will fit neatly with the bottom in the footwell and the back resting on the edge of the seat with a seatbelt around it. When the barometer has arrived at its destination, it should be raised slowly to the vertical and the transportation preparations described above undone with care. Move it slowly to where it will hang and fix it to the wall. Barometers are heavy, so use a secure fixing. If you have an accident with them and mercury is spilled, contact the local poisonous-waste-disposal expert. Aneroid Barometers The aneroid barometer responds to variation in air pressure by the flexing of an evacuated bellows against the tension of a powerful spring. The movement of the bellows is amplified by a system of levers and converted into a rotation by a chain wrapped around a pivoted rod to which a pointer is attached. The chain is kept in tension by a hairspring, which also takes the slack out of the lever joints. Variations in air pressure can give about 200 degrees of movement of the pointer. Aneroid barometers require no special precautions when moved, and many were designed as pocket barometers and altimeters for surveyors and climbers. Published and © by John Locke 1996