With a rising column the convexity is visibly greater than with a falling column. The larger the bore of the tube the less the correction for capillarity. A tube with a bore of less than 0.25 inch should be avoided. Inasmuch as the meniscus of the larger tube has a narrower range, the readings during changes are a little more accurate with a tube of larger bore.
When a barometer is new, the surface of the mercury is very bright. In the course of two or three years—or less—the surface of the mercury in the cistern may become oxidized, turning gray. Although unsightly, this condition offers no material interference with accurate reading. In time, also, the vacuous part of a poorly constructed barometer may acquire a gray tint owing to the use of impure mercury in filling the tube and cistern.[1] Although this may not affect the reading appreciably, it is a mark of careless workmanship, and such an instrument should be sent to a reputable maker to be refilled with clean mercury.
After a few years of service the film on the surface of the mercury may require cleaning. Emptying, cleaning, and refilling a barometer tube is a delicate task even for trained experts; it should not be attempted by one without experience.
A clean room, free from dust, is desirable for barometers. Dust is not preventable, but it should not be permitted to accumulate on instruments. A soft, damp—not wet—cloth will remove and gather it without scattering; a camel’s hair brush will remove it from corners and crevices which the cloth does not reach. If glass cylinder and tube are clean and bright there need be but little error in setting the mercury to the scale, or in cutting the top of the meniscus sharply by the sliding windows.
If a barometer is to be removed from its support the mercury in the cistern should first be raised until the mercury in the tube is flush with the opening near the top of the case. If it is to be
- ↑ Pure mercury in a saucer made chemically clean leaves no stain or metallic film when shaken about. If it contains even a trace of other metal—lead, zinc, or tin—spots and streaks will adhere to the saucer. If the mercury is not freshly distilled it may contain moisture. Mercury may be freed of its own oxide by filtering through a clean paper funnel with a pin-hole perforation at the bottom. The mercury used in Weather Bureau barometers is chemically pure.
