The Glow-Tube

Author: J.B. Hoag

If a trace of gas, say argon, is admitted to a tube containing two cold metal plates, and a battery is connected as in Fig. 18 A, a current can be made to pass through the tube and a glow of light will appear on the electrodes.

Fig. 18 A. A glow-tube circuit

Starting at zero and increasing the battery voltage, it will be found that a certain minimum voltage, called the striking potential V_s must be applied across the tube before the discharge will start. The value of V_s is different for different tubes and gases.

Fig. 18 B. Striking potential, Vs, for helium at different pressures. (From E. & N. P.)

Figure 18 B shows the effect of changing the pressure of the gas in the tube.

The time for the discharge to start, after a voltage equal to or greater than V_s has been applied, is usually very small, a few micro-seconds, especially if the applied voltage is well above V_s. If, however, the electrodes are contaminated or if surface charges exist, the time lag may amount to several minutes. The time for the discharge to cease after the voltage has been removed varies from a few micro-seconds to a few milliseconds.

When the tube is glowing, the applied voltage may be reduced below the striking potential, to the extinction voltage V_x, before the discharge will cease.

This is illustrated in Fig. 18 C, where the electric current through the tube has been plotted vertically and the applied voltage horizontally.

Fig. 18 C. Striking and extinction voltages of a glow-tube. (From E. & N. P.)

A glow-tube may be used around a high-frequency oscillator to determine whether the circuit is oscillating or not, and to locate the regions of greatest r.f. voltage. The higher the voltage, the brighter the glow in the tube.

A glow-tube may also be used to produce " relaxation " oscillations. In a relaxation circuit, a battery sends current through a resistor R into a condenser C, charging it rapidly at first, then more and more slowly. This requires a length of time which depends upon RC (see The Time Constant). The greater RC, the longer the time required. A glow-lamp, connected across the condenser, is in a non-conductive state until the voltage across the condenser rises to its striking potential. Then the glow discharge suddenly starts, and the condenser suddenly discharges until the voltage has dropped to the extinction potential of the glow-tube. The process then repeats itself. The frequency of the pulsations can be varied over wide limits by changing R and C.