Because of the positive coefficient of resistivity of tungsten, incandescent lamps permit a current considerably greater than the normal lamp current to flow for a short initial period. When lamps are used as load for testing electrical apparatus, the factors affecting this inrush must be considered. Size of power source, its distance from the load (lamps), and frequency of switching the lamps (employing repetitive operations for endurance testing) are evaluated.From the given curve the necessary cooling time for the desired current inrush may be determined, and thus the frequency with which the lamps could be switched ``on'' and ``off'' may be calculated. Practical means for obtaining the required cooling time are described.