A hand boiler or (less commonly) love meter is a glass sculpture used as an experimental tool to demonstrate vapour-liquid equilibrium, or as a collector's item to whimsically "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.
The liquid inside a 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. 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.
Sometimes a hand boiler is used to show properties of distillation. Since the liquid both evaporates and condenses at relatively cool temperatures while in an enclosed system, the boiler can be turned upside down, and the top end can be placed in ice water. The gaseous form of the liquid will condense in the cooled chamber. Since the liquid is often colored with dye, but the dye does not evaporate or condense at the same temperature, the liquid that condenses in the cooled chamber is colorless, leaving the pigment behind.
In popular culture, hand boilers used to be sometimes 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 colored red. Love meters were a common collector's item or a souvenir. Depending on how the item was packaged, one would grasp the lower bulb to "prove" how passionate one was, or a couple would each grasp one end to see who would force the liquid into the other's bulb.
Hand boilers are much more commonly used as a scientific novelty today.