Resistance in a Light Bulb (globe).

Most recent answer: 12/12/2011

Q:
I have a light globe rated 60 Watts and it runs on a 240 Volt AC supply. I believe it then uses 0.25 Amps,ie watts = Amps x Volts so Amps = Watts/Volts. However, I measured the resistance of the globe as 80 Ohms so using the formula Volts = Amps x Resistance I get Amps equal to 3 this way, ie 240/80. Where am I in error?
- Michael Sobb (age 70)
Rydalmere New South Wales Australia
A:
Dear Michael,
Your result is about right.   The resistance of tungsten increases by about a factor of 20 between room temperature, where you measured it, and about 3000o Kelvin, when the bulb is lit.   Take a look at:  for a table of values. 
This is why you get a brief current surge when you turn on the lights, before the filament gets hot.

LeeH

(published on 12/12/2011)