The history of Digital hygrometers: 

Perhaps Leonardo—a man born in Italy in the 15th century—was the first to come up with an instrument to measure water vapor in the air. He put a dry cotton on one side of the sky. Then he placed an object of exactly the same weight as cotton on the other side of the sky. As the dry cotton absorbs water vapor from the air, it becomes heavier and this side of the sky begins to fall. The difference between the two weights is a measure of humidity.

nhiệt kế

nhiệt kế

Scientists measure relative humidity using an instrument called a "psychrometer." A "psychrometer" is created by two thermometers tied together. A thermometer's bubble is wrapped in water-soaked material. To begin measuring relative humidity, rotate the psychrometer until the wrapped thermometer maintains a stable temperature, which must be lower than the dry thermometer. The actual air temperature is measured by the thermometer that is being dried. The difference between the two temperatures is called "wet-bulbdepression" and is the result of the evaporation of water from the substance. Scientists record the temperature and "wet-bulbdepression" from a low-dry thermometer, and then make a graph to calculate relative humidity. This is also the working principle of the wet and dry bulb hygrometer.

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