A lactometer is used for measuring the density (creaminess) of milk, a saccharometer for measuring the concentration of sugar in a liquid, or an alcoholometer for measuring the ethanol content in spirits. The number read off the scale is not always density but also derived quantities. There are many different hydrometers available depending on the use. This method is based on the Archimedes principle. If a hydrometer is immersed in a glass of water, it would sink deeper than it would in a glass of syrup because syrup is denser than water. The hydrometer is immersed in the sample and the density of the sample can be read directly from the scale: The deeper it sinks, the less dense is the sample. Hydrometers and the hydrostatic balance are both based on the Archimedes principle.įor more information on the relationship between mass and density, see here.Ī hydrometer is a floating glass body with a bulb filled with a metal weight and a cylindrical stem with a scale. This experiment helped Archimedes discover the relation of mass to volume. The crown was less dense than pure gold, so the gold had to have been mixed with another metal. It had the same mass, but more volume – that means lower average density. The crown displaced more water than the gold bar, the water went up higher – so the crown obviously had more volume. He immersed both objects in a water container. In order to verify the gold content of King Hieron’s crown, he took the crown and a bar of pure gold of the exact same weight as the crown. We all know what he did next: he ran across town – naked – yelling: ‘Eureka!’. He realized that the overflowing water stood in direct relation to the immersion of his body volume into the water. (Okay, this part of the story is probably made up but read on to see what he found.) When he sank into a very full bathtub, it started to overflow. Exhausted from all the pondering over this difficult task, he took a bath. Archimedes, a great mathematician, physicist, engineer, inventor and astronomer, was asked to prove – or disprove – that Hieron’s crown consisted of nothing but gold, without destroying it.Īrchimedes went home to think about the problem. The goldsmith was accused of having added some silver to the crown. In a nutshell, King Hieron II had a crown made of pure gold.
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