The device looks like a sealed glass tube with a weighted bottom. A thin stem with number markings extends upward from the weighted bulb. When you place it in a liquid, the tool sinks into the fluid.
It stops sinking when the displaced liquid equals its own total weight. This behavior follows the ancient physics rule known as Archimedes’ principle. A denser liquid can support more weight in a smaller volume.
Therefore, the heavy glass instrument will float much higher in thick syrups. A less dense liquid like rubbing alcohol lets the tool sink deeper. Students often think the tool measures the actual weight of the liquid.
Instead, it measures the ratio of the liquid density compared to pure water. You take the reading right where the liquid surface crosses the thin stem. This simple design gives you the specific gravity without any complicated math.
Temperature changes the density of all liquids slightly. Because of this, you must check the liquid temperature before reading the scale. Most instruments are calibrated to work perfectly at exactly 20 degrees Celsius.