||The side arm attached to the outer jacket of this experimental SON lamp contains a small ball bearing. A constriction just before the opening of this side arm prevents the bearing from falling into the main outer jacket of the lamp.Despite this example having been provided by the Preston incandescent lamp factory of Thorn Lighting, it is clear from its construction that it is not a Thorn product - indeed it would appear to have actually been made by Osram at Shaw. Quite how it made its way to Thorn, and the precise purpose of the lamp remains unknown!
It is suspected, however, that the ball in the side tubulation may have been used to determine the quality of the vacuum in the lamp's outer jacket by a non-destructive method. There exists a technique by which a ball of this type may be magnetically spun around the circumference of a narrow diameter glass tube such as this, and its speed of rotation can be determined by measuring the rate at which it interrupts a laser beam incident on the glass. The angular velocity of the ball decreases with increasing gas pressure in the tube owing to increased frictional resistance of remaining impurities in the vacuum. Therefore, for a calibrated system it is possible to determine vacuum pressure by this technique.
Determination of the vacuum quality in a lamp's outer jacket is a rather important measurement to be able to take, since the luminous efficacy of the arc tube is strongly dependent on the level of the vacuum. Residual gases or vapours in the outer jacket conduct and convect heat away from the arc tube causing its temperature to fall, and with this loss of thermal energy comes a loss in luminous efficacy as well.