Hygro-, baro-, and thermometers (dial-style)

You might've seen one of these in someone's house, or on a boat. It's a mini weather station consisting of a hygrometer, barometer, and thermometer. The hygrometer measures the relative humidity of the air, barometer measures the air pressure, and thermometer obviously tells you how warm (or cold) it is.

But how do they work? What turns the little dial on the indicator and what acutally measures the moisture, pressure, and temperature? Well, it's all down to really basic physics.

The hygrometer

The hygrometer uses a measuring element - this can be any material that expands when humidity increases, and contracts when humidity decreases. The hygrometer measures and shows the change in length of this element. Since the element is attached to the dial (through a number of levers and axles), the dial turns as humidity changes.
The measuring element is usually a human hair, thread, or paper.

The barometer

The barometer uses a metal capsule called a diaphragm. When air pressure increases, the diaphragm is pressed together, and when air pressure decreases, the diaphragm expands - sort of like a balloon being deflated or inflated. The change in the diaphragm is transformed into a turn of the dial on the meter's face with the help of levers.

The thermometer

You might know thermometers that use mercury or other liquids inside them. When the temperatur of the liquid increases, it expands and moves up in the tube. But dial-type thermometers use a spiral spring made of two different metals. The metals contract with decreasing temperature and expand with increasing temperature, but to different extents. Because the one is less reactive than the other, the spring makes accurate, defined rotating movements. This movement is directly transferred to the dial which indicates what the temperature is.