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.
VI. THEORIES OF “OVERSHOOTING.”
The theory given by Taylor to account for the “overshooting” of tungsten lamps is based on the fact that there is a small amount of residual gas in the lamp, which is attracted to the walls of the lamp when it is cold; and when the lamp is lighted and warms up, this residual gas is driven off lowering the vacuum. With a high vacuum, practically all the energy must be radiated from the filament; conversely, on a lowering of the vacuum, some of the heat is carried away by convection and conduction. When all the heat is carried away by radiation the filament runs at a higher temperature and will give more light.
Another theory is that a cold tungsten filament lamp absorbs and occludes certain gaseous substances from the low pressure space within the chamber. Owing to the presence of these gases the filament shines more brightly when first brought quickly to incandescence, but after the gases have been driven off by the heat, the extra luminescence disappears and can be regained only by prolonged cooling and rest.
Still another theory, and the one that seems the most logical to the writer, is that the increase of resistance accompanying the rise of temperature takes a certain small interval of time so that when the temperature is rising at the rate of thousands of degrees per second, the resistance lags perceptibly. The resistance does not suppress the current as quickly as it should and an extra rush of current and heat energy goes through the filament, raising the temperature above normal, with a corresponding increase in brilliancy.
VII. AMOUNT OF “OVERSHOOTING”.
In order to determine the amount of over shooting, the writer has made photographs as shown below. Number one was made with the lamp under voltage, number two by using normal voltage and suddenly turning the lamp on by means of a snap switch, thereby obtaining the overshooting, and finally number three was made by impressing voltage above normal. The pictures were obtained by using the photographic arrangement as before described. All three prints were made from the same film, that is the three pictures were made upon one film thus insuring the same development and printing for all. The print has been cut merely to allow a closer comparison of the intensities.
