Most of the cheaper "professional" touch probes are based on the original Renishaw design (the basis of their present success), which essentially has a normally-closed switch that opens when the probe stylus touches anything causing a microscopic movement. The opening of the circuit can be sensed in several ways, some of which involve a ground connection. You can buy quite low-cost probes now based on the same principle. Generally use a wired connection relying on power from the machine.

Renishaw have much more advanced systems now often using wireless or optical and the probes can be mixed with tools in an ATC carousel, and also probe whilst rotating. Correspondingly MUCH more expensive!