The singular property of the tungsten lamp to “overshoot” or to give temporarily a higher initial than normal candle power, was first discovered by John B. Taylor and is explained in the following manner:—The filament of the carbon incandescent lamp possesses a negative temperature coefficient; that is to say, a rise in voltage causes a more than corresponding rise in current and when the lamp is connected to a source of constant potential, the current starts at a comparatively small value and increases to a maximum when the lamp has attained full candle power.

In the case of the tungsten lamp, the situation is just the reverse, since tungsten has a positive temperature coefficient. When the lamp is connected to a constant potential supply the current is a maximum when the lamp is cold and decreases to a final value when the lamp reaches full brilliancy. The most important difference between the two lamps due to these different characteristics is that while a tungsten lamp reaches full candle power the instant the current is turned on a carbon incandescent lamp comes up to full candle power only after a perceptible period of time.

The apparent temporary increase in the candle power of a tungsten lamp was observed early after the lamp was invented but it was generally ascribed to some possible physiological action due to the slow contraction of the pupil of the eye.

The following curve was obtained by means of the oscillograph and shows clearly the rush of current for the first instant after the lamp is turned on. The break in the curve is due to an imperfection in one of the operating switches and has nothing to do with any characteristic of the lamp. The cycle wave was put on merely to obtain the time.

In order to prove that this overshooting occurs, an actual photograph of the intensity has been made. This was obtained by making a box 1´×1´×3´ absolutely light proof and arranging a lamp inside so that it could be turned on and off at will. A slit, fitted with a shutter, was cut in one end of the box which permitted the light to fall upon a revolving oscillograph film. The film holder was attached to the box with thin metal strips and revolved by means of a small motor. As is seen the whole arrangement was nothing more than a large camera.

The following photograph shows the phenomenon quite clearly, point A denoting where the lamp was turned on.