The instrument that measures the magnitude of a force is called a force gauge or dynamometer.

Here's a little more about them:

* Force Gauge: This is a generic term for any instrument that measures force. There are many different types, each designed for specific applications and ranges of force.

* Dynamometer: This is a specific type of force gauge that typically uses a spring to measure force. The spring stretches or compresses in proportion to the applied force, and this displacement is then read on a scale.

Let me know if you'd like to know more about different types of force gauges!