The Strobotron

Author: J.B. Hoag

It is possible to control the rate of charge of a condenser and its discharge through a glow-lamp so as to produce comparatively large amounts of light in short pulses of predetermined duration and spacing. A special tube for this purpose is known as a strobotron. As shown in Fig. 18 D, it contains a cold cathode of cesium compound (to assist in starting the arc) and an anode, an inner grid of wires and an outer grid made of a graphite ring. The tube is usually filled with neon gas at about 1.5 cms. of mercury pressure.

Fig. 18 D. A simple stroboscope circuit using a strobotron tube

In the circuit of Fig. 18 D, the large condenser C (say 4 μfd.) is filled through the comparatively low resistance R (a few hundred or thousand ohms). It will then suddenly discharge through the strobo-tron to produce a brilliant but very brief flash of light provided the voltages on the grids are such as to permit the arc to strike in the tube. A pulse oscillator, whose frequency can be altered, is connected to the inner grid and serves to control the rate at which the flashes occur. In one commercial form of the apparatus, called a Strobotac, the flashing rate can be varied throughout the range from 600 to 14,400 per minute. Although each flash lasts only a few (about 10) micro-seconds, the currents through the strobotron are so large (several hundred amperes) that sufficient light is emitted to be useful to illuminate objects in bright daylight.

The pulses of light are used to study vibrating and rotating machinery by the well-known stroboscopic principle and for high speed motion-picture work. With single-flash circuits, the tubes are used for short-stop photography of objects moving at high speed.