page_title: Galvanometer
section_title: Tangent galvanometer
hierarchical_section_title: Galvanometer / Tangent galvanometer
caption_attribution_description: Drawing of tangent galvanometer from around 1890, made by J.H. Bunnel Co. Base 7 in. diameter. This was a type known as a 'Western Union standard'. It has 5 separate coils of resistance ~0, 1, 10, 50, and 150 ohms. The 7/8 in. compass needle in the center can have a silk fiber suspension.
context_page_description: A galvanometer is a type of ammeter. It is an instrument for detecting and measuring electric current. It is an analog electromechanical transducer that produces a rotary deflection, through a limited arc, in response to electric current flowing through its coil. The term has been expanded to include uses of the same mechanism in recording, positioning, and servomechanism equipment.
context_section_description: A tangent galvanometer is an early measuring instrument used for the measurement of electric current. It works by using a compass needle to compare a magnetic field generated by the unknown current to the magnetic field of the Earth. It gets its name from its operating principle, the tangent law of magnetism, which states that the tangent of the angle a compass needle makes is proportional to the ratio of the strengths of the two perpendicular magnetic fields. It was first described by Claude Servais Mathias Pouillet in 1837.
  
  A tangent galvanometer consists of a coil of insulated copper wire wound on a circular non-magnetic frame. The frame is mounted vertically on a horizontal base provided with levelling screws. The coil can be rotated on a vertical axis passing through its centre. A compass box is mounted horizontally at the centre of a circular scale. It consists of a tiny, powerful magnetic needle pivoted at the centre of the coil. The magnetic needle is free to rotate in the horizontal plane. The circular scale is divided into four quadrants. Each quadrant is graduated from 0° to 90°. A long thin aluminium pointer is attached to the needle at its centre and at right angle to it. To avoid errors due to parallax a plane mirror is mounted below the compass needle.
caption: Tangent galvanometer made by J.H.Bunnell Co. around 1890.
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