Elements of physical manipulation / by Edward C. Pickering.

  • Pickering Edward C. (Edward Charles), 1846-1919.
Date:
1874-1876
Catalogue details

Licence: Public Domain Mark

Credit: Elements of physical manipulation / by Edward C. Pickering. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    133/248 (page 117)
    dently due to a column of air of this height, whose weight is pro- portional to p. Hence —dp — apd H, dH— or H = ^ log p + C. The constant a equals the pressure due to a column of air of height unity and under pressure unity, or its recijjrocal equals 60,300. The elevation HJ, or difference in height of two points ^and H' is therefore H — H' = 60,300 (log/ — log p). In order to obtain the true height by the formula, it is necessary to apply several corrections, of which the most important are the following. I. Capillarity. The effect of this force is to depress the mer- cury column by an amount dependent on the diameter of the tube. A constant quantity should therefore be added to each reading. Unfortunately this result is modified by the adhesion of the liquid to the tube, which renders this correction uncertain; sometimes, therefore, the height of the meniscus or curved portion is allowed for, but the best way is to use a very large tube, when the effect of capillarity becomes inappreciable. II. Temperature of the Mercury. The standard jsressure as- sumes the temperature to be 0° C.; at higher temperatures the mer- cury would be lighter, and the pressure less. Let p be the observed height at temperature T, and P the true height with mercury at zero, then p = P (1 + «^'), or P = ^ji, in which a equals the expansion of the mercuiy per degree. As the scale expands also, allowance must be made for this, which gives a = .00009, when the scale is of brass. The temperature T is given by the thermometer attached to the instrument, and this correction should always be applied to a mercurial barometer, but not to an aneroid, when the height is wanted. III. Temperature of the Air. In the above discussion the air also is supposed to be at zero. If warmer if will be lighter, and the elevation greater than that here assumed. Call t and t' the temperatures above and below, and their mean t — h (t f). The true elevation HJ' will then equal PJ {1 + « t) which a = the coefficient of expansion of air. This is the most im- portant correction of all, and should always be applied, or large errors will be introduced.