This is a pH probe (or pH electrode). It's a Pinpoint brand electrode from American Marine, Inc., purchased from a mail-order pet supplier. It cost $45 in 2007. Note that the delicate glass bulb at the tip is still enclosed in the shipping cover, which protects it and keeps it wet. You can see that some of the salt in the buffer solution inside the shipping cover has creeped out at the edge of the cover. The cable is 10 feet (3 meters) long. The connector at the end is male BNC connector.

The pH probe should be kept away from light to prevent algae growth. Mine sits in a gray perforated PVC tube at the top corner of the aquarium. Readings will be more accurate if there is some water flow in the vicinity of the probe because you'll get a better representation of the average conditions in the tank rather than those in a quiet corner. It does make a difference.

The probe should be cleaned and recalibrated periodically. I use a soft artist's paint brush to gently clean the delicate probe tip at each water change. Don't ever let the probe tip dry out! This is most likely to happen during water changes.