Capitalisation of "Hall effect"

You should properly name it the Hall effect.

For more info read this Wikipedia article to understand how it should be used in sentences. Examples:

At the beginning of a sentence:

The Hall effect was discovered in 1879.

In the middle of a sentence:

One very important feature of the Hall effect is that it differentiates between positive charges moving in one direction and negative charges moving in the opposite.

At the end of a sentence:

The absence of the free transverse boundaries renders the interpretation of the Corbino effect simpler than that of the Hall effect.

Used as an adjective in a sentence:

Hall effect devices produce a very low signal level and thus require amplification.