There isn't a single device specifically called a "density meter". Instead, density is typically measured using a combination of tools depending on the substance and the desired accuracy. Here are some common methods:

For Liquids:

* Hydrometer: This simple device floats in a liquid and its calibrated scale indicates the liquid's density. It's commonly used for measuring the specific gravity of liquids.

* Pycnometer: A small flask with a precisely known volume used to determine the density of liquids. The liquid is filled to a mark, weighed, and the density calculated from the mass and volume.

* Density Meter: This is a more sophisticated device that uses various techniques like oscillating U-tube or vibrating tube to measure the density of liquids. It's highly accurate and commonly used in industrial settings.

For Solids:

* Water Displacement Method: This simple method involves submerging a solid in a graduated cylinder filled with water. The change in volume indicates the volume of the solid. The density is then calculated using the mass and volume.

* Pycnometer: This method can also be used for solids. The solid is placed in the pycnometer, and the volume of the solid is calculated by the difference in volume filled with water with and without the solid.

* Archimedes' Principle: By measuring the buoyant force acting on a solid submerged in a fluid, you can calculate its density.

For Gases:

* Gas Density Meter: This device measures the mass of a known volume of gas, allowing for the calculation of its density.

* Mass Spectrometer: While not specifically for density, a mass spectrometer can be used to determine the composition of a gas, which can be used to calculate its density.

The best device for measuring density depends on the specific substance, the required accuracy, and the available resources.