There isn't a specific instrument that directly measures inertia. Inertia is a property of matter, not something that can be directly measured with a device.

However, we can infer inertia through observing an object's resistance to changes in motion. Here's how:

* Mass is a measure of inertia: The more massive an object, the greater its inertia. We can use scales to measure mass, which indirectly gives us an understanding of inertia.

* Observing acceleration: Newton's second law of motion (F = ma) connects force, mass, and acceleration. By applying a known force to an object and measuring its acceleration, we can calculate its mass and therefore its inertia.

So, instead of a single instrument, we use various tools and principles to understand and quantify inertia:

* Scales for measuring mass

* Force sensors for measuring applied forces

* Accelerometers for measuring changes in motion

Ultimately, inertia is a fundamental property of matter that we infer through observation and measurement of related quantities.