A Valve-operated Time Switch
W. R. Gray · British Journal of Radiology · 1933
The mechanical time switch of the present day is often a marvel of ingenuity. The designer, however, starts off with a very big handicap: the elimination of the effects of inertia. It is to this factor more than anything else that errors in fractional second exposures must be attributed. The principle of the time switch forming the subject of this article is the electrical control of the exposure time by means of a filament relay tube (gasfilled three-electrode valve), used in conjunction with suitable values of resistance and capacity. To follow the scheme in detail, it is necessary that the characteristics of the filament relay tube should be completely understood. With a filament relay tube connected in a circuit such as Fig. 1, provided the grid is sufficiently negative (with respect to the cathode) and an A.C. tension is applied between cathode and anode, no current can pass. Without such a bias on the grid, or with a bias too low in value, a current will flow every half-cycle. It is important to remember, however, that once the anode tension is sufficient to start the discharge through the tube, the action of the grid is completely ineffective. The grid cannot become negative whilst the gas discharge lasts. It will, however, react immediately the anode tension falls again to zero. Thus in the filament relay tube we have a means of interrupting any circuit in which the tube might be included by merely controlling the rate at which the grid is negatively charged.