Glow tube

Glow tube is a tube in which the bulb contains gas at a low pressure. The electrodes –cathode and anode-nay be of different construction. The glow tube usually has a cold cathode covered with the material of low work-function.

For working conditions the anode is made more and more positive till the ionization of gas begins. The initial ionization is a result of the action of a number of natural ionization sources (cosmic rays etc.). The discharge at a low pressure is known as glow discharge, which is electrically characterized both by a low current density and by high voltage drop.

The voltage, at which ionization begins, is referred to as the striking potential.

The additional number of electrons for maintaining the discharge is produced as a result of the cathode emission due to the positive-ion bombardment. During the discharge only a small part of the cathode is covered with glow for low values of total current.

In fact, as the current through the tube increases the portion of the cathode surface that is covered with glow increases linearly with the current.

This indicates that the current density and potential drop in the glow tube remain constant.

Glow tubes are chiefly used to stabilize the output voltage from mains units.

This is because the voltage drop is largely independent of the current. The gas, typically neon or argon, is usually at a pressure of 0.1mm of mercury.

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Electronic equipment

Industrial equipment of electronics is known to play a very important role today.

Hundreds of electronics equipments are now available to science¹ and various industries to help do jobs better or more economically or to take over jobs² that could not be done otherwise.

The application, use, and proper maintenance³ of the many electronics equipments now in industrial use demand a certain knowledge to be of the fundamentals⁴ of various standard electronics equipments.

We already know that in motors, incandescent lamps⁵, transformers⁶, etc., the electricity always flows in the copper wire or other metal parts. But consider lightning, where electricity seems to jump through space⁷. The great electric pressure of lightning forces the electric current to pass through the air. In the same way⁸, inside any radio tube, tiny electric currents are made to pass through the space separating certain parts in the tube. Such action where electricity appears to the flow through space instead to being confined to metal conductors or circuits — is said to be electronic.

Why is it called electronic? Years ago, scientists who were trying to explain how electricity passed through space, imagined such an electric current to be a steady stream of tiny electrical particles. They called these particles electrons. Today, any electric current is believed to consist of countless number of electrons. Only when electricity passed through space, when the stream of electrons comes out of the metal into the open⁹, is such action said to be electronic. For a device to be called electronic, electricity must flow across the space inside the device and be controlled by that device.

In ordinary air, electrons can be made be jump through space only by pressure of high voltage. But if it enclosed in a tube from which the air has been removed, the electrons flow across the space more easily. All tubes must be carefully sealed for the desired conditions to be maintained inside the tube. Most of the small tubes are vacuum tubes¹⁰, the large ones usually containing mercury or other vapour.

Some electric lights are electronic. The common incandescent light bulb is not considered to be electronic even though it is enclosed like a radio tube, for the electric current flows entirely within the metal filament¹¹. In contrast, the fluorescent lamp is electronic, its light is produced by the action of electric current flowing through the space between the two ends of the lamp.

Electronics as a science is not new, for radio, sound pictures, fluorescent light, etc. are known to depend upon electronics.

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Diodes

The simplest combination of elements constituting an electron tube is the diode. It consists of a cathode, which serves for emitting the electrons, and a plate or anode surrounding the cathode, which acts as a collector of electrons. Both electrodes are enclosed in a highly evacuated envelope of glass or metal. If the cathode is indirectly heated, there must be a heating spiral or a heater. The size of diode tubes varies from tiny metal tubes to large-sized rectifiers. The plate is generally a hollow metallic cylinder made of nickel, molybdenum graphite, tantalum or iron.

A basic law of electricity states that like charges repel each other and unlike charges attract each other. Electrons emitted from the cathode of an electron tube are negative electric charges. These charges may be either attracted to or repelled from the plate of a diode tube, depending on whether the plate is positively or negatively charged.

Actually, by applying a potential difference (voltage) from a battery or other source between the plate and cathode of a diode, an electric field is established within the tube.

The lines of force of this field always extend from the negatively charged element to the positively charged element. Electrons, being negative electric charges, follow the direction of the lines of force in an electric field.

By establishing an electric field of the correct polarity between cathode and plate and “shaping” the lines of force of this field in certain paths, the motion of the electrons can be controlled as desired. A battery is connected between plate and cathode of a diode, so as to make the plate positive with respect to the cathode, the lines of force of the electric field extending in a direction from the cathode to the plate.

Again, applying a heater voltage results in emission of electrons from the cathode. The electrons follow the lines of force to the positive plate and strike it at high speed. Since moving charges comprise an electric current, the stream of electrons to the plate is an electric current, called the plate current.

Upon reaching the plate the electron current continues to flow through the external circuit made up of the connecting wires and the battery. The arriving electrons are absorbed into the positive terminal of the battery and an equal number of electrons flow out from the negative battery terminal and return to the cathode, thus replenishing the supply of electrons lost by emission.

As long as the cathode of the tube is maintained at emitting temperatures and plate remains positive, plate current will continue to flow from the cathode to the plate within the tube and from the plate back to the cathode through the external circuit.

Now a battery connection has been reversed so as to make the plate negative with respect to the cathode. When voltage is applied to the heater the cathode will emit a flow of electrons. However, these electrons are strongly repelled from the negatively charged plate and tend to fill the inter-electrode space between cathode and plate. Since no electrons actually reach the plate. Since no electrons actually reach the plate, the tube acts like an open circuit.

The total number of electrons emitted by the cathode of a diode is always the same at a given operating temperature. The plate voltage (voltage between plate and cathode) has no effect, therefore, on the amount of electrons emitted from the cathode. Whether or not these electrons actually reach the plate, however, is determined by the plate-to-cathode voltage, as well as by a phenomenon known as space charge.

The term space charge is applied to the cloud of electrons that is formed in the interelectrode space between cathode and plate. Since it is made up of electrons, this cloud constitutes a negative charge in the interelectrode space that has a repelling effect on the electrons being emitted from the cathode. The effect of this negative space charge alone, therefore, is to force a considerable portion of the emitted electrons back into the cathode and prevent others from reaching the plate.

The space charge, however, does not act alone. It is counteracted by the electric field from the positive plate, which reaches through the space charge to attract electrons and thus partially overcomes its effects. At low positive plate voltages only electrons nearest to the plate are attracted to it and constitute a small plate current. The space charge then has a strong effect on limiting the number of electrons reaching the plate.

As the plate voltage is increased, a greater number of electrons are attracted to the plate through the negative space charge and correspondingly fewer are repelled back to the cathode. If the plate voltage is made sufficiently high, a point is reached eventually, where all the electrons emitted from the cathode are attracted to the plate and the effect of the space charge is completely overcome. Further increases in the plate voltage cannot increase the plate current through the tube. And the emission from the cathode limits the maximum current flow.

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