arm length

Balanced arm lamps

A balanced-arm lamp is a lamp with an adjustable folding arm which is constructed so that the force due to gravity is always counteracted, regardless of the position of the arms of the lamp. Many lamp brands (such as the Anglepoise and Luxo L-1 as well as other devices, such as drawing boards, use this principle.


These lamp arms invite comparison with a human arm: the lamp cap is the hand, and there are three joints (equivalent to the wrist, elbow and shoulder) which connect it to the body, in this case reprsented by the base. Just as with the human arm, the advantages of the three joints are that the arm can reach virtually any point in space within a hemisphere with a radius equal to the arm's length. The same mechanism can be employed in other devices with similar requirements, such as copy holders for typists and some computer display holders. This article uses the terminology lamp cap, forearm, upper arm and base for the four basic parts of these lamps.

For the physics and theory behind balanced-arm lamps, see Classical mechanics, linkage and torque. There are different methods to balance the lamp-cap of a balanced-arm lamp. Some lamps have two coil springs working in parallel on both sides of the pivoting arm. (A set of springs functions in the same way as a single spring.) Others are balanced with counterweights (a method frequently used in drawing-boards). Friction between parts of the lamp arm can also be used to maintain balance. There are a number of mechanical solutions (coupling, hydraulics and pneumatic arms) which have occasionally been used for balanced-arm lamps.

Lamps balanced with springs

There are many variations of construction with springs. Springs can be located on the mechanical equivalent of the forearm or the upper arm, or both, as well as nearer the base. Some lamps use tension springs, and others use compression springs. The image at the left shows (left to right) a compression spring at rest, then under load, followed by a tension spring at rest, and then under load. Springs have a limited lifting capacity and extension length. Some springs can resonate, producing low-level unwanted noise.

One Tension spring

Two Tension springs

Compression springs

Other types of spring

Lamps balanced by pressure and friction

Lamps balanced with one counterweight

A Advantages of one swinging counter balance is that the lamp can have a longer arm length. Disadvantages is that the stand is less stable, the lamp need a heavy foot to stand stable.

Lamps balanced with two counterweights

Lamps using other systems

Here are some less common ways of balancing arms.

In popular culture

Two spring balanced desk lamps of this type feature as the main characters in the Pixar animated film Luxo Jr..


See also

External links

Patents history

Classifications IPC :F21V21/26

International patent category : B23B31/171

  • GB191104491 An Improved Device for Supporting or Suspending Electric Lamps and the like (1911)
  • - 1921 harmonica arm -
  • 1923 parallelogram & counter weight
  • 1926 friction
  • 1929 2 counter weight
  • 1934 spring and counter weight
  • 1937 friction
  • 1938 2 parallelogram 2 springs
  • 1939 2 pressure springs
  • 19.. a-symmetric arm-lamp
  • 1947 friction and a spring
  • 1949
  • 1949 spring in the arm
  • 1954 friction arm

  1. -- 1961
  2. 1971
  3. - 1991 -
  4. EP0518569 1992
  5. EP1274545 2003 mechanical coupling devices

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