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Description: | 100 FEET 1,000 FEET 10.000 FEET ALT 45,000 FEET XIVB/1327 1393/41 (scales and manufacturer's typeand serial numbers). PATENTED (stencilled in white on the back of the instrument) AM and crown (Air Ministry mark on the back of the instrument). MANU Kollsman Instrument Company. 1941 Merrimack, Newhampshire, The United States of America. DES KOLLMAN,Paul. 1928 Merrimack, New Hampshire, The United states of America. DES after TORRICELLI,Evangelista. 1643 Florence, Italy. CAUTION! this object is emitting radiation at a rate of 2 counts per second from the radium doped paint on the bezel. However, this is well below the 30 maximum permissible counts per second, which is roughly equivalent to 7.5 micro Sieverts per hour.
This is a Kollsman barometric altimeter also known as a "Kollsman window" built to Royal Aiforce Specifications. It is in effect an accurate aneroid barometer that has altidute marked on its clock-like dial.
An aneroid is a small flexible metal vacuum cell rather like a concertina bellows, in this case probably a phosphor-bronze alloy. At MSL (mean sea level) the standard air pressure is calculated to be 1013.25 hPa or millibars, at which the instrument should read zero feet. Air pressure causes the cell to inflate as the pressure drops and deflate when the pressure increases. Atmospheric pressure decreases by approximately 1 millibar for an increase in altitude of 27 feet or 8.23 meter. Therefore by linking the aneroid to the needles by a system of levers and gears the change in the dimension of the aneroid can be linked to the pointer on the diall which can measure altitude accurately to 45,000 feet i.e. 13 716 meters. There is a knob on the front of the instrument that can calibrate the instrument using the QNH, the regional or local air pressure at sea level, so that the instrument would read zero at the QFE, height above ground at a particular part of the flight such as at an airfield or the top of a mountain range. It was the invention of this device and the Sperry artificial horizon that allowed James "Jimmy" Doolittle to make the first blind fliying trials, in 1929, thus making flying in cloud or at night a much safer proposition.
The design and construction of the aneroid , with a highly conductive alloy, automatically adjusts the mechanism for changes in temperature. Typically in the troposphere the temperature drops approximately 3 degrees Celsius or 5.4 degrees Fahrenheit per 1000 feet or 305 meters rise in altitude. The troposphere is regarded as the first 11 Km or 36000 ft of the atmosphere. Above this height the temperature remains fairly constant at -55 degrees Celcius or -67 degrees Fahrenheit.
This instrument was used by field geologists to make fairly accurate determinations of height above MSL depending on the local weather conditions. Local airpressure can at times vary wildly from the standard over time so frequent recallibration is require for reasonably accurate measurement. However, with ordinace survey maps with contour lines giving accurate height measurements at fixed positions this is not too difficult. | License: | http://www.hmag.gla.ac.uk/spirit/rights/ | Publisher: | Hunterian Museum and Art Gallery, University of Glasgow | Rights holder: | Hunterian Museum and Art Gallery, University of Glasgow | Subjects: | SCIENTIFIC COLLECTION : | Source: | Hunterian Museum | Creator: | Hunterian Museum and Art Gallery, University of Glasgow | Identifier: | http://www.huntsearch.gla.ac.uk/cgi-bin/... | Language: | en-GB | Go to resource |
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