Why does a bulb from a network park bulb stays on and off a precise period of time ,for both on and off modes?

Question

Sitting on a bench,i've noticed that a light bulb is switching on and off with some regularity . So i've started counting with the help of my phone. The result made me ask this question, because it always stays on for 21.1 sec and off for 18.4 sec. I did the counting for a while and always the same result. WHY? Why a precise period of time? A very scientific answer i need...

Answer 1

The light has a broken wire in its ballast transformer.  When the ballast is cool, the wire makes contact, completing the circuit.  As the ballast heats up it expands, eventually breaking the contact and opening the circuit, turning the light off.  The 21.1 sec is the current heat-up time, and the 18.4 sec is the current cool-down time.  Since the break in the circuit draws an arc, melting a minute portion of wire each time it opens the circuit, the heat-up time will gradually shorten and the col-down time will gradually lengthen, until  the light eventually fails completely.