Connectors on pH electrodes
pH probes basically act as batteries, just with very high internal resistance (giga-Ohm range). With such high internal resistance the signal is very "weak", so shielded cable is used to reduce interference. BNC connector also shields the signal, so it is a good choice. Making a measurement circuit is not trivial. For example: you have to take PCB leakage into consideration.

The probe voltage is too low and its internal resistance is too high to be directly read with a multimeter.