A wooden wall plaque supports a brass outer casing which protects the glass barometer tube and wood and leather cistern. A brass screw provides the means to raise or lower the level of mercury in the cistern to a fiducial point in the form of an ivory pointer. Air pressure on the free surface of the mercury in the cistern supports the column of mercury, the length of which is read by a vernier operated by a rack and pinion over a graduated scale marked on the casing. A mercury thermometer attached to the casing gives the temperature of the barometer, and allows corrections to be made. The Fortin Barometer was commonly used at meteorological stations to measure atmospheric pressure.
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A portable mercury barometer that was invented in by the French instrument maker Jean Nicholas Fortin — In other mercury barometers air pressure is calculated from the distance between the level of the mercury in the reservoir and in the tube. In the Fortin barometer the bottom of the reservoir is flexible originally it was made from leather and can be raised or lowered by means of a screw, allowing the surface of the mercury to be adjusted to a predetermined level.
A vernier scale on the side of the barometer tube is then lowered until its base touches the top of the mercury in the reservoir and a reading is taken from the fixed scale marked on the tube. Before travelling, the base of the reservoir is raised using the adjusting screw until both reservoir and tube are filled with mercury. The Fortin barometer makes no correction for changes in temperature, which affect the volume of mercury, or for capillarity, which introduces inaccuracies. The instrument is nevertheless sufficiently accurate for most purposes.
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Fortin barometer A mercury barometer that requires the accurate setting of the mercury level at a fixed point scale zero, see fiducial point. The reading of the mercury height is then taken by adjusting a vernier scale to the top of the mercury column. The barometer was invented early in the nineteenth century by Jean Fortin. See also Kew barometer.
The Fortin Barometer
In the Fortin barometer, the level of mercury in the glass at the bottom of the barometer cistern is adjusted to a scale zero, known as the fiducial point, each time a reading is to be taken. The level of mercury in the column is then read against the scale, using a vernier adjustment for extra accuracy. Air is evacuated from the top of the tube of mercury and the lower end is fixed in the cistern containing the reservoir of mercury. The Fortin barometer is simple to use as it has a clear easy-to-read linear scale. These barometers can be mounted on a wall or suitable pillar. In addition to instruments manufactured under the Russell Scientific Instruments' name, we manufacture, service and repair F Darton and WF Stanley, Griffin and George, and Gallenkamp precision mercury barometers. Fortin barometers can be moved without risk of damage but should always be carried flat or upside down.
294 - Fortin Mercury Barometer