Wheel or "Banjo" Barometers are mercury column barometers operating with a "J" tube as shown in the picture. These are properly called wheel barometers. They have also been nicknamed banjo barometers due to their shape. The hand on the dial is moved by the float in the mercury column rising and falling with changes in air pressure. The float is tied on a string that goes over the wheel and is held taught by a counterweight. As the wheel turns, the black hand is moved over the dial giving the reading. The other hand (brass) is moved by turning the knob below the dial or the knob mounted on the glass. This is a memory crutch so that you can see the change in pressure.


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