A hand boiler or love meter is a glass sculpture used as an experimental tool to demonstrate Charles's Law and vapour-liquid equilibrium or as a collector's item to measure love. It consists of a lower bulb containing a volatile liquid and a mixture of gases that is connected usually by a twisting glass tube that connects to an upper or "receiving" glass bulb.
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The coloured liquid inside a hand boiler is composed of a volatile mixture of liquids that places the boiling point of the mixture to just above room temperature. [1] The liquid inside the Hand Boiler does not actually boil. The “boiling” is caused by the relationship between the temperature and pressure of a gas. As the temperature of a gas in a closed container rises, the pressure also rises. The molecules are moving faster in the warmer gas. There must be a temperature (and pressure) difference between the two large chambers for the liquid to move. When held upright (with the smaller bulb on top), the liquid will move from the bulb with the higher pressure to the bulb with lower pressure. As the gas continues to expand, the gas will then bubble through the liquid, making it appear to boil. The fact that the liquid is volatile (easily vaporized) makes the hand boiler more effective. Adding heat to the liquid produces more gas, also increasing pressure in the closed container.
In popular culture, hand boilers are often known as love meters because the tube that separates the upper and lower bulbs is twisted into a heart shape and the volatile liquid is coloured red. Love meters are a common collector's item or a souvenir.