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The prodigious experiment of a hand boiler

It’s amazing, simply by holding the device in your hand, you can make water boil? Is it magic? Actually, this could be used as a very crude blood pressure estimator. As there is a more or less direct relation between hand temperature and blood pressure, a person with low blood pressure will take a lot longer to “boil” the water than someone with normal or high blood pressure. But there are so many exceptions to the relation between blood pressure and hand temperature that a hand boiler cannot be used as a scientific instrument.

For years, these hand-boilers were used as “Love meters”, where they were placed in people’s hands and the faster the water boiled, the more love there was… But the science behind it is quite simple. The upper and lower bulbs of the device are at different temperatures, and therefore the vapor pressure in the two bulbs is different. Since the lower bulb is warmer, the vapor pressure in it is higher. The difference in vapor pressure forces the liquid from the lower bulb to the upper 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.


Hand boilers date back at least as early as 1767, when the American scientist Benjamin Franklin encountered them in Germany. He developed an improved version in 1768 after which they were called Franklin's pulse glass.

There are a few Franklin Pulse Glasses in the HealthWorks Collection. Discover them, and other amazing medical devices on


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