Mercurial Tube Barometers
Variations in atmospheric pressure which can be due to weather, or height above sea-level, cause an indicating needle to move up or down a scale representing the high pressure associated with fine, dry weather, or the low pressure asociated with rain and storms.
The barometer dial ranges from '28' - '31' . These represent inches of the fluid metal mercury in a glass tube in the mercurial barometer. A more scientific scale ranges from '920' - '1060' millibars, found in nautical barometers. The legends : 'Rain, Change, Fair' etc. found on many dials are really decoration only, as can be seen on this antique porcelaindial illustrated. The needle on a barometer dial will not be zipping backwards and forwards charting the progress of every fluctuation of weather conditions.
It is not critical where the needle falls on the dial. You 'read' the small variations the needle makes, up or down, that can be interpreted as the likely weather to be expected in the next few hours. The needle will dwell mostly in the same spot anyway.Behind the flap at the back is the mercury tube with 'U' bend below. There should be a hollow glass tube beside with a glass counterweight (can sometimes be a broken half of an old thermometer) held with silk thread. Above is a grooved pulley-wheel that accommodates a turn of silk and an opposite turn of silk thread that descends to a glass 'float' tube that sits in the open end of the mercury tube. The pulley wheel has the dial indicator hand attached. The silk can easily get detached from the pulley during transit. Set the pointer to correspond with a reading you know to be accurate (perhaps the thread has broken and been re-tied shorter at some stage, throwing the needle off the scale). If the needle is pointing to the 'change' area this is as accurate as you need. Height above sea level would only be sigificant if you were in a mountainous situation.
The barometer needle will indicate the changes in weather to be expected 24 hours ahead. It is therefore best used as a tool to predict the likelyhood of any change in the weather.
You will notice that in the centre of the dial, there is a second needle pointer that can be moved around the dial by turning an ivory knob below the dial. You turn the ivory knob below the dial to move the pointer to cover the position of the needle. This will reveal the movement of the needle.This has no functional effect on the barometer mechanism itself. Its purpose is to be moved so that the pointer covers the active needle of the barometer itself. In this way, the active needle can be seen to have moved (or not) from day to day. This indicating pointer may be adjusted to exactly shadow the progress on the dial of the active needle
It is this pointer that is so vital in showing that the barometer needle has changed for otherwise it would be necessary to remember mentally each tiny variation of the indicating needle. Using the pointer as a marker will show whether the pressure is going up, or down.
One useful way to see if the pressure indicating needle is falling or rising, is to tap the glass gently with a finger. The needle will be observed to move up or down by a small fraction, showing which way the pressure is going.
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