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What is a Negative Cathode?

Geisha A. Legazpi
Geisha A. Legazpi

A negative cathode is one of two primary electrodes in vacuum tubes. It releases negative ions that are attracted to the positive anode. A negative cathode may be either a hot or cold cathode. While a hot cathode is usually heated by a tungsten filament wrapped around it, a cold cathode is not heated directly but instead eventually reaches elevated temperatures during its operation. Even if a cold cathode is not directly heated, it may operate in high temperature because the temperature inside the enclosure will keep rising when current flows through the cathode.

The hot and negative cathode is used in the cathode ray tube (CRT), a special vacuum tube. CRTs are used for oscilloscopes and tube-type television (TV) screens. The positive side of the CRT is energized by as much as 25,000 volts (V) on the screen side. A hot cathode at the rear of the tube is inside an enclosure referred to as the gun. The electron from the hot cathode is emitted and finds its way on the inside of the screen after being accurately deflected by a magnetic or electric field in the vertical and horizontal axis.

A CRT TV containing negative cathodes.
A CRT TV containing negative cathodes.

A magnetron is a negative cathode device used for generating and amplifying radio frequency at elevated frequency bands called microwave. It uses a metallic confinement of special geometry referred to as a cavity. The cavity, along with a permanent magnetic field, generates microwave energy that can be used for radar and microwave ovens.

One variation of the cold cathode is used for lighting. By choosing different phosphor coatings, the ions generated by the cold cathode may produce a variety of colors. It is a common practice to cite the color of the cathode.

Cathode ray tubes use negative cathodes.
Cathode ray tubes use negative cathodes.

Batteries operate with a positive cathode and a negative anode. The cathode material for a rechargeable battery is able to adapt with repeated charge and discharge cycles. Negative battery charges arrive at the cathode material and onto the load for the battery. The polarity of the cathode is specified based on the charges that reach it.

A diode is a solid-state device that acts as a one-way valve for electrical charges. The negative cathode is made up of a negative (N)-type semiconductor, which has free electrons, and an anode is made up of a positive (P)-type semiconductor that has a lack of electrons. The schematic symbol of a diode indicates the anode as an arrow pointing into the cathode, and the direction of the arrow indicates the opposite of electron flow, which is referred to as hole flow.

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    • A CRT TV containing negative cathodes.
      By: Gino Santa Maria
      A CRT TV containing negative cathodes.
    • Cathode ray tubes use negative cathodes.
      By: Vasiliy Timofeev
      Cathode ray tubes use negative cathodes.