This is a CENCO model 76890 mercury barometer made by the Central Scientific Company of Chicago, Illinois. Based on the design of the CENCO mounting board I believe it was made in the 1940s, it is shown in the 1941 CENCO catalog but has a different design in the 1950 catalog. It is in working condition and was at the inexpensive end of CENCO's mercury barometer models, it lacks the frills present in the Henry J Green observatory barometer elsewhere in this website. Key design features include:

• The barometer is Fortin in spirit since it has an indicator stylus in the cistern for adjustment of the cistern mercury level. However it does not have a screw adjustment for mercury level at the base of the cistern. Rather the cistern, which is made of clear plastic, is screwed up or down in its entirety to raise or lower the mercury level. This avoids the use of a flexible leather seal in the cistern, which can fail with time. However adjustment by turning the entire cistern is not as smooth and precise as the screw adjustment in a classic Fortin barometer. Also the plastic cistern is somewhat clouded with age making zero-point adjustment difficult. They called this the "CENCO-Improved Design" which they patented in 1927 (US Patent 1,632,084).
• It has a 4 mm diameter tube bore. With this diameter capillarity of the mercury in the glass tube can affect measurements. The H J Green observatory barometer has a 0.25 inch, or 6.35 mm, bore. NBS Monograph 8, available elsewhere on this website, recommends a minimum bore of 0.25 inches for accurate measurements.
• It has a vernier scale but lacks a rack and pinion mechanism to move the scale. The scale is merely slid up and down manually, which is not as accurate. The scale is in inches Hg and centimeters Hg, and has divisions to allow readings to 0.005 inch. There is a conversion table plaque on the middle of the mounting board for millibars. However the zero point of the vernier scale lacks an indicator at the back of the glass tube, making parallax error possible when taking readings. The scale extends down to 24.5 in Hg, which makes it usable to elevations of about 4,000 feet above sea level.
• The barometer glass tube is entirely enclosed in a hexagonal brass tube, except for the reading window for the vernier scale. This makes it less breakable than other mercury barometer designs.
• A mercury thermometer is mounted on the hexagonal brass tube for barometer temperature measurements.

It's not in the same class as the Henry J Green observatory barometer, but a working mercury barometer for $159 within driving distance was too good to pass up.

A copy of US Patent 1,632,084 for the CENCO-Improved Design may be downloaded below (3 pages):

Go to CENCO catalog page.

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