The Sylvania P607 is a fairly common UV sensitive photocell designed for use in flame detection systems. Flame detectors are typically used in boilers and furnaces to detect the presence of a flame in the combustion chamber. Flame detectors also see some use in fire suppression systems, where they can be used to sound an alarm or activate a sprinkler system when a flame is detected. The internal construction of the P607 is extremely simple and contains only two small loops of wire; one loop acts as a light sensitive cathode, and the other receives the electrons which are released from the cathode when it is struck by UV light. The P607 is designed to operate from a 600V AC line frequency power source, and as such the cathode and anode would reverse position every time the current inverted polarity.
Boilers and furnaces have long lives and as a result, this device is still in common use. Honeywell produces a part, the Honeywell 18838, that is generally considered a safe replacement for this tube.