Power rating formula

Resistance of a dc circuit element depends upon its rating in accordance with the relation,

Using above relation we can say

(a) A bulb of thin filament has more resistance.

(b) A low wattage bulb has more resistance (filament thinner) than a high wattage bulb of same rated voltage.

(c) Glow of a bulb ∝ H ∝ I². In series a low wattage bulb glows more than a bulb of higher wattage while in parallel a high wattage bulb glows more than a bulb of lower wattage.