The resistance of a heater coil is less than that of electric bulb filament. When heater is switched on in parallel, more current start flowing through the heater coil and current through the bulb filament decreases making it dim. After some time, when heater coil becomes hot its resistance increases. As a result,current through the heater coil decreases and the current through the bulb filament increases and thus dimness of the bulb decreases.
