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Positive temperature coefficient of Tungsten.

Whoever told you that the current increases was wrong. The current decreases but it doesn't follow Ohm's Law. As a rough rule of thumb, a ten percent drop in voltage will reduce current by 5%. If you think about it the negative resistance characteristic would make nonsense of current regulating a heater.

Your supposition that the filament is not a constant resistance is correct. Tungsten like most other metals has a positive temperature coefficient of resistance of about 4.5 x 10 (-3) K(-1) which is a jargon laden way of saying its resistance increases by nearly half a percent for each degree (Celsius or Kelvin) temperature rise.

When you underheat the heater it reduces in temperature therefore the resistance drops.


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