You're likely asking about an instrument used to determine the polarity of something. There are several instruments depending on what you want to measure the polarity of:

For electrical polarity:

* Multimeter: This is a versatile tool that can measure voltage, current, and resistance, all of which can be used to determine the polarity of an electrical circuit.

* Polarity Tester: This is a specialized tool designed specifically to identify the positive and negative poles of a battery or electrical circuit.

* Oscilloscope: Used for visualizing waveforms and determining the polarity of AC signals.

For chemical polarity:

* Spectrophotometer: This instrument measures the amount of light absorbed or transmitted by a substance, which can be used to determine the presence and orientation of polar molecules.

* NMR Spectrometer: Nuclear magnetic resonance spectroscopy is used to study the structure and dynamics of molecules, including the presence of polar bonds.

For magnetism:

* Compass: This tool is used to determine magnetic north, which is directly related to the polarity of the Earth's magnetic field.

* Magnetometer: This instrument measures the strength and direction of magnetic fields, which can be used to determine the polarity of magnetic materials.

Please provide more context about what you want to measure the polarity of, and I can give a more specific answer.