Photocell---Ballest

[RobFurn]

Please see the following pic. Its a wiring diagram for a photocell (dusk till dawn job) Can anyone tell me what the "Ballast" bit of the circuit does. From what I understand Ballest in electrical circuits is simply a resistor, but I cant think wht I would need to limit the current. The lamp is simply a standard outsde half lantern job with energy saving bulb fitted
Thanks

 

[Spark123]

That there looks like a street light circuit!!
The ballast is used primarillary with an ignitor to cause the high voltage necessary to start a discharge lamp. It's second purpose is to limit the current through the lamp once it has started.
As far as you are concerned, your lamp will simply go between Lo and N.

 

[RobFurn]

thought it would be something like that
Cheers 123