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A barretter, also known as a ballast tube, is a type of resistor device historically used in electrical circuits, particularly in early automotive and airplane applications.


  1. Resistor Type:
  • A barretter is typically constructed using a fine iron-wire resistor element.
  • This resistor element is enclosed within a sealed glass bulb or tube.
  1. Filling and Environment:
  • The glass bulb is filled with a gas, often hydrogen.
  • The presence of hydrogen helps regulate the thermal characteristics of the resistor, influencing its resistance properties.


  1. Temperature-Dependent Resistance:
  • The primary function of a barretter is to provide a resistor whose resistance changes with temperature.
  • As the current through the barretter increases, it heats up due to the electrical resistance of the iron-wire element.
  1. Stabilizing Current:
  • The temperature-dependent nature of the barretter’s resistance acts as a self-regulating mechanism.
  • When connected in a circuit, the barretter adjusts its resistance to maintain a relatively constant current flow, even if there are variations in the applied voltage.


  1. Automotive Use:
  • In early automotive electrical systems, barretters were used to stabilize the current flow through lighting circuits and other electrical components.
  • They helped prevent excessive current surges that could damage bulbs and other sensitive electrical equipment.
  1. Aircraft Use:
  • Similarly, in aircraft electrical systems, barretters were employed to ensure stable operation of critical electrical devices.
  • They were particularly useful in maintaining consistent current supply under varying operational conditions, including changes in engine speed and electrical load.


  1. Self-Regulation:
  • Barretters provided a simple yet effective method for stabilizing current without the need for complex electronic regulation circuits.
  • Their inherent design allowed for reliable operation over a wide range of operating conditions.
  1. Durability:
  • Encapsulation within a sealed glass bulb protected the iron-wire resistor from environmental factors and mechanical damage, enhancing durability in harsh conditions.

Evolution and Modern Usage:

  1. Obsolete Technology:
  • With advancements in semiconductor technology, barretters have largely been replaced by more efficient and precise electronic voltage regulators and current limiters.
  • Modern automotive and aircraft electrical systems rely on solid-state regulators and sophisticated control circuits for precise current and voltage management.
  1. Historical Significance:
  • Despite their obsolescence in modern applications, barretters played a significant role in early electrical engineering and contributed to the development of stable and reliable electrical systems.

A barretter, or ballast tube, was an early form of temperature-compensating resistor used to stabilize current flow in electrical circuits, particularly in automotive and aircraft applications. Its operation relied on the temperature-dependent properties of an iron-wire resistor housed within a hydrogen-filled glass bulb. While no longer in widespread use due to advancements in semiconductor technology, barretters remain an important historical innovation in electrical engineering for their role in early electrical system stability and reliability.

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