Incandescent lamp resistance behavior: Which statement best describes the resistance characteristics of a typical incandescent filament lamp?

Introduction / Context:
Filament lamps are strongly temperature-dependent resistors. Understanding their cold versus hot resistance helps in surge analysis, dimmer design, and failure diagnosis (inrush currents at switch-on can be many times the steady current).

Given Data / Assumptions:

• Tungsten filament with positive temperature coefficient of resistance.
• Supply is within rated voltage so filament reaches normal operating temperature.
• No series ballast assumed unless specified.

Concept / Approach:

At room temperature (cold), filament resistance is low. When energized, current heats the filament to thousands of kelvin; resistance increases several-fold (hot resistance). Therefore, the device exhibits both a cold resistance (initial) and a hot resistance (operating), and these values are markedly different.

Step-by-Step Solution:

Verification / Alternative check:

Measure resistance with an ohmmeter (cold) and compute operating resistance from V/I at rated operation (hot). Hot resistance often measures ~10× the cold resistance, confirming the dual behavior.

Why Other Options Are Wrong:

• “cold resistance” or “hot resistance” alone ignores the other operating condition.
• “ballast resistance”: applies to fluorescent/LED drivers, not incandescent filaments.
• “zero resistance”: no practical conductor has zero resistance; filament certainly does not.

Common Pitfalls:

• Underestimating inrush current and inadequately rating switches or dimmers.
• Using cold ohmmeter readings to predict running current without accounting for heating.

Final Answer:

both cold and hot resistance