A balanced-arm lamp, sometimes called a floating arm lamp, is a lamp with an adjustable folding arm which is constructed such that the force due to gravity is always counteracted by springs, regardless of the position of the arms of the lamp. Many lamp brands (such as the Anglepoise, originator of the concept, and Luxo L-1), as well as other devices, use this principle.

Configuration

The five terms

This article uses the terminology: * ''lamp cap'', * ''forearm'', * ''upper arm'', * ''stand'' or ''body'' and * ''base'' for the five basic parts of these lamps.

The general design

A balanced-arm lamp has a ''base'', a ''stand'' or ''body'', (in most cases) ''two'' connected ''arms'' (in many cases fitted with springs), and a ''lamp-head''. The lamp can be moved into almost any position, and the balancing device will maintain the position until moved again. The same overall mechanism can be employed in ''other'' devices with similar requirements, such as: * copy holders for typists, * microphone holders in studios or * computer display holders. For the

physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

and theory behind balanced-arm lamps, see

Classical mechanics Classical mechanics is a Theoretical physics, physical theory describing the motion of objects such as projectiles, parts of Machine (mechanical), machinery, spacecraft, planets, stars, and galaxies. The development of classical mechanics inv ...

, linkage and

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

. There are different methods to balance the arms and the lamp-cap: *

coil spring A tension coil spring A coil spring is a mechanical device that typically is used to store energy and subsequently release it, to absorb shock, or to maintain a force between contacting surfaces. It is made of an elastic material formed into the ...

s
In some cases a set of ''two'' coil springs working in parallel on both sides of the pivoting arm is used.
(They function in the same way as a ''single'' spring.) *

counterweight A counterweight is a weight (object), weight that, by applying an opposite force, provides balance and stability of a machine, mechanical system. The purpose of a counterweight is to make lifting the load faster and more efficient, which saves e ...

(s)
(a method frequently used in

Sash window A sash window or hung sash window is made of one or more movable panels, or "sashes". The individual sashes are traditionally paned windows, but can now contain an individual sheet (or sheets, in the case of double glazing) of glass. History ...

s and

drawing board A drawing board (also drawing table, drafting table or architect's table) is, in its antique form, a kind of multipurpose desk which can be used for any kind of drawing, writing or impromptu sketching on a large sheet of paper or for reading a la ...

s). *

Friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

between parts of the lamp arm, * Coupling or * hydraulics or pneumatic arms.

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 to the base. Some lamps use tension springs, and others use compression springs. The adjacent image 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

Spring-balanced upper arm, flexible forearm

This lamp has a stiff flexible tube as its forearm, and a spring-balanced arm to support it near the middle. That way there is a flexible lamp with a long reach.

Tension spring over wheels

The wheel to control the forearm is smaller than the wheel that controls the upper arm. The lamp cap rotates the same direction as the upper arm and forearm.

Two arms, one spring, one toothed wheel

With this technique the tension spring has a double function: it controls both the forearm and the upper arm. This is not a common arm lamp system. The lamp cap rotates the same direction as the upper arm and forearm.

Two tension springs

Two parallelograms and two tension spring sets

One tension spring set controls the forearm, which is a

parallelogram In Euclidean geometry, a parallelogram is a simple polygon, simple (non-list of self-intersecting polygons, self-intersecting) quadrilateral with two pairs of Parallel (geometry), parallel sides. The opposite or facing sides of a parallelogram a ...

having two bars. A stronger spring set controls the upper arm, which has three parallel bars (see photo, left). (There is a less stable construction with two parallel bars in the upper arm which is less expensive to manufacture.) Due to the parallel linkage of the lamp, the lamp cap keeps pointing in the same vertical direction when adjusting the height of the lamp. As with most balanced-arm lamps, the whole assembly can rotate in the horizontal plane, and is mounted on a stable foot.

One parallelogram and two extension spring sets

A lamp like the Anglepoise lamp has one

and two extension springs. One spring controls the shortest arm of a parallelogram. Parallel to this shortest arm there is the forearm. To keep these arms parallel there are two other parallel arms (upper arm) that are controlled by a second extension spring. The lamp cap rotates the same direction as the upper arm and forearm.

Single forearm bar, one parallelogram, two tension spring sets

The forearm of this lamp consist of a single bar. When it is adjusted, the angle of the lamp cap changes along with the arm.

Tension springs within the arms

A lamp like the

Tolomeo desk lamp The Tolomeo incandescent lamp, incandescent desk lamp is an icon of Italy, Italian modern industrial design, design. It was designed by Michele De Lucchi and Giancarlo Fassina in 1986 for the Artemide company. It won the Compasso d'Oro design pri ...

has no

s in either arm. In this modern lamp the tension springs are hidden in the arms. The lamp cap rotates the same direction as the upper arm and forearm.

Compression springs

Compression springs in the arms

The short arms (green) stay parallel. One spring puts pull

force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...

on the blue arm. (If the blue arm is tilted back the second spring on this arm gets push force.) The blue arm controls the two parallel arms (red) that make up the upper arm. The other spring puts pull force on the other blue arm. This arm controls the forearm (magenta). The two springs can be the same size; one spring has to lift more

weight In science and engineering, the weight of an object is a quantity associated with the gravitational force exerted on the object by other objects in its environment, although there is some variation and debate as to the exact definition. Some sta ...

but is more vertical. The other spring lifts less weight but is more horizontal.

Compression springs near the foot

One compression spring controls a very short arm (grey). The

Roller chain Roller chain or bush roller chain is the type of chain drive most commonly used for transmission of mechanical power on many kinds of domestic, industrial and agricultural machinery, including conveyors, wire- and tube-drawing machines, pr ...

cable Cable may refer to: Mechanical * Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof * Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...

connects this arm parallel to the forearm (yellow). The longer spring controls the (blue) arm. The body (red) can turn in the horizontal plane. The body is connected to a stable foot. The body can be smaller because the pressure springs can protrude below the body.

A compression spring with one parallelogram

In this method, just one compression spring controls a pantographic arm.

Lamps balanced by pressure and friction

Rubber between two "parallel" arms

In this construction (commonly found in Italian lamps) gravity causes the parallelogram to deform such that the gap between the long arms becomes smaller. This in turn increases pressure on the

rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Types of polyisoprene ...

in the gap, and the resulting

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

holds the arm in place.

Friction from twisting

The

between the twisted arm controls the movement between the arms.

Friction between the ''upper'' arm and the ''fore''arm

This arm system also works with friction.

Lamps balanced with counterweight

Lamps balanced with ''one'' counterweight

Advantages of one swinging counterbalance is that the lamp can have a longer arm length. Disadvantages are that the lamp is inherently less balanced, thus it needs a heavy stand to remain stable.

A single counterweight

This construction uses a chain to keep the arm with the

and the forearm parallel. The lamp-cap and counterweight move in opposite directions (both away from the base or both towards it). The balance that is required is shown by the following formula: :d₁ = Lamp-cap to base; m₁ = weight of lamp-cap :d₂ = counterweight to base; m₂ = weight of counterweight :m₁ × d₁ = m₂ × d₂

One parallelogram with one counterweight

This lamp has a heavy base, with a parallelogram

pantograph A pantograph (, from their original use for copying writing) is a Linkage (mechanical), mechanical linkage connected in a manner based on parallelograms so that the movement of one pen, in tracing an image, produces identical movements in a se ...

construction formed by bars which are all hinged on each other. One long bar extends past the parallelogram construction and has a

. One short bar also extends past the parallelogram construction and holds the lamp cap. The base has an arc formed stand so the counterweight does not hit the stand in vertical position.

One parallelogram with an extended counterweight

Pantographic parallel arms balance a light-weight-lamp and counter-weight

This lamp is similar to the one above, but as the counterweight has a low position the lamp needs no heavy foot for

stability Stability may refer to: Mathematics *Stability theory, the study of the stability of solutions to differential equations and dynamical systems ** Asymptotic stability ** Exponential stability ** Linear stability **Lyapunov stability ** Marginal s ...

as the center of gravity is low. The counterbalance does not swing, but slides and rotates safely around the center of the stand.

Three parallelograms with one counterweight

This lamp stand has a complex construction with matching narrow openings. This lamp is out of production.

Lamps balanced with ''two'' counterweights

A single arm with two counterweights

With this lamp the movement is very limited (the arm moves up and down only) but the technique is nice to look at. As the forearm becomes more horizontal the

ceases to be balanced on both side of the base. The weight of the base is needed to avoid over-balancing.

Two arms with two counterweights

A lamp type like the Tizio works like this. A balancing arm (the forearm) has on one side of an axis a small

and on the other side a lamp cap. These parts are lifted by a second bigger balancing arm (the upper arm) that has on the other side of a second axis a heavier counterweight. The lamp designed by Edouard-Wilfrid Buquet in 1927 works in this way. The lamp is in the collection of the

Museum of Modern Art The Museum of Modern Art (MoMA) is an art museum located in Midtown Manhattan, New York City, on 53rd Street (Manhattan), 53rd Street between Fifth Avenue, Fifth and Sixth Avenues. MoMA's collection spans the late 19th century to the present, a ...

. The two axes have a different angle, and this has some advantages. Lamp-cap x fore-Arm = Arm x Small weight (Lamp-cap + Small weight) x upper-arm = Arm x Big weight. Big and small counterweight arm-lamp

Lamps using other systems

Here are some less common ways of balancing arms.

Lamps having a vertical axis

This kind of lamp has a forearm balanced by weight or springs. Its disadvantage is that the upper arm cannot point up or forwards.

Mechanical coupling devices

This lamp works by

coupling A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mo ...

together the upper arm and the forearm. It is called a gravity-locked or cam-locked elbow.

Hydraulic arm

The pivoting arm

streetlamp A street light, light pole, lamp pole, lamppost, streetlamp, light standard, or lamp standard is a raised source of light on the edge of a road or path. Similar lights may be found on a railway platform. When urban electric power distribution b ...

s in the Schouwburgplein, Rotterdam use a pair of

hydraulic cylinder A hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in construction equipment ( engineering vehicles ...

s to control the forearm and upper arm.

Pneumatic arm

Arm lamp with a

pneumatic cylinder Pneumatic cylinder, also known as air cylinder, is a mechanical device which uses the power of compressed gas to produce a force in a reciprocating linear motion. Like in a hydraulic cylinder, something forces a piston to move in the desired ...

. The gravity compress the air in the pneumatic cylinder.

In popular culture

* Two spring-balanced desk lamps of this type feature as the main characters in the 1986

Pixar Pixar (), doing business as Pixar Animation Studios, is an American animation studio based in Emeryville, California, known for its critically and commercially successful computer-animated feature films. Pixar is a subsidiary of Walt Disney ...

short film '' Luxo Jr.''. It also serves as the letter I in the Pixar logo presented before and at the end of each of their films. * Myster Shadow-Sky is the only composer who from 1980 to 2007 performed the balanced-arm lamp as a musical instrument: the archisonic lamp. In 1980 he wrote the score "Ludus Musicae Temporarium" for a balanced arm lamp orchestra. The world premiere was performed by Les Percussions de Strasbourg at the Manca contemporary music festival in Nice in 1988. He released five albums with a four spring balanced arm lamp musical instrument (and more with other musical instruments). They are available for free download. Referenced in the book "Les chercheurs de sons" (2004), as the unique creator of a balanced arm lamp as a musical instrument.

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
GB206435
1923 parallelogram & counter weight
GB254972
1926 friction
GB285045
1929 2 counter weight
GB481198
1934 spring and counter weight * FR757890 1934 CARWARDINE GEORGE * FR784932 1935 CARWARDINE GEORGE
GB470502
1937 friction
GB489547
1938 2 parallelogram 2 springs
GB504791
1939 2 pressure springs
GB536342
19.. a-symmetric arm-lamp
GB589994
1947 friction and a spring
GB618344
1949
GB629364
1949 spring in the arm
GB713378
1954 friction arm * 1961
GB1249774A
1971 * 1991 * EP0518569 1992 * EP1274545 2003 mechanical coupling devices

References

{{Artificial light sources Light fixtures