Bartrum's barometric compensator

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Bartrum's barometric compensator
Manufacturer	Bartrum
Production years	20th century
Production location	(unknown)

Variations of barometric pressure affect the time of swing of a pendulum in two ways. High pressure, leading to high air density, buoys up the pendulum, reducing the effective value of gravity and so slowing the pendulum; the high density also increases the frictional drag and produces a further slowing effect. The first effect can be calculated fairly accurately, and for a pendulum with brass bob amounts to about 1/5th second per day for each 1 inch rise of mercury barometer; the second effect depends upon the shape of the pendulum bob, but for a cylindrical bob is roughly equal to the first, so that the total error is about 2/5th sec./day for 1 inch rise of pressure. Bartrum’s compensator, first describe by him in 1929, is intended to compensate this effect.

[edit] How it works

It consists of a pile of vacuum boxes (as employed in an aneroid barograph). The bottom of the pile is fixed to the pendulum rod, while the top carries a weight. A rise of barometer causes the boxes to contract under the increased pressure and so to lower the weight. If the compensator is mounted well above the mid-point of the pendulum rod, lowering the weight will speed it up, and the amount of speeding can be adjusted to compensate the slowing described above. The actual movement of the weight is about 1/40th inch for 1 inch change of barometric pressure.

[edit] Memories

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[edit] In the Science Museum's Records

Inv. No: 1948-340