page_title: Galvanometer
section_title: Edward Weston
hierarchical_section_title: Galvanometer / History / Edward Weston
caption_attribution_description: English: Drawing of D'Arsonval Galvanometer (ammeter) movement, of Weston type, from around 1900. This is the mechanism used in most moving-pointer (galvanometer) meters today. Only the pole pieces of the horseshoe-shaped magnet are shown, and part of the left-hand pole piece is broken out by the artist to show the coil. It consists of a light coil of wire with an attached pointer, suspended on jewel bearings between the poles of a magnet, with a spiral hairspring to return it to zero. When a current is passed through the coil, it creates a magnetic field that opposes the magnet's field, creating a torque that rotates the coil against the restoring force of the spring. The coil turns through an angle until the restoring force of the spring is equal to the force of the coil. Since the angle of deflection is proportional to the force, which is proportional to the current, the deflection is proportional to the current. The screwdriver slot at the top of the axis is for adjusting the meter to zero. Alterations: removed caption, and faint background discoloration.
context_page_description: A galvanometer is an electromechanical instrument used for detecting and indicating an electric current. A galvanometer works as an actuator, by producing a rotary deflection, in response to electric current flowing through a coil in a constant magnetic field. Early galvanometers were not calibrated, but their later developments were used as measuring instruments, called ammeters, to measure the current flowing through an electric circuit.
  Galvanometers developed from the observation that the needle of a magnetic compass is deflected near a wire that has electric current flowing through it, first described by Hans Christian Ørsted in 1820. They were the first instruments used to detect and measure small amounts of electric currents. André-Marie Ampère, who gave mathematical expression to Ørsted's discovery and named the instrument after the Italian electricity researcher Luigi Galvani, who in 1791 discovered the principle of the frog galvanoscope – that electric current would make the legs of a dead frog jerk.
  Sensitive galvanometers have been essential for the development of science and technology in many fields.
context_section_description: Edward Weston extensively improved the design. He replaced the fine wire suspension with a pivot, and provided restoring torque and electrical connections through spiral springs rather like those of a wristwatch balance wheel hairspring. He developed a method of stabilizing the magnetic field of the permanent magnet, so the instrument would have consistent accuracy over time. He replaced the light beam and mirror with a knife-edge pointer that could be read directly. A mirror under the pointer, in the same plane as the scale, eliminated parallax observation error. To maintain the field strength, Weston's design used a very narrow circumferential slot through which the coil moved, with a minimal air-gap. This improved linearity of pointer deflection with respect to coil current. Finally, the coil was wound on a light-weight form made of conductive metal, which acted as a damper. By 1888, Edward Weston had patented and brought out a commercial form of this instrument, which became a standard electrical equipment component. It was known as a "portable" instrument because it was affected very little by mounting position or by transporting it from place to place. This design is almost universally used in moving-coil meters today.
  Initially laboratory instruments relying on the Earth's own magnetic field to provide restoring force for the pointer, galvanometers were developed into compact, rugged, sensitive portable instruments essential to the development of electro-technology.
caption: D'Arsonval/Weston galvanometer (ca. 1900). Part of the magnet's left pole piece is broken out to show the coil.
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