D'Arsonval Galvanometer

  The two French inventors of this form of galvanometer in the early 1880s came from quite different backgrounds. Jacques D'Arsonval (1851-1940) was a director of a laboratory of biological physics and a professor of experimental medicine, and one of the founders of diathermy treatments. Marcel Deprez (1843-1918) was an engineer and an early promoter of high-voltage electrical power transmission.

   In the D'Arsonval-Deprez design the coil has many turns of fine wire, and is suspended by flat ribbon of wire which serves as one lead-in wire. The connection to the lower end of the coil is provided by a light, helical spring that provides the restoring torque. The electro-magnetic torque is greatest when the magnetic field lines are perpendicular to the plane of the coil; this condition is met for a wide range of coil positions by placing the cylindrical core of soft iron in the middle of the magnetic gap, and giving the magnet pole faces a concave contour. Since the electro-magnetic torque is proportional to the current in the coil and the restoring toque is proportional to the angle of twist of the suspension fiber, at equilibrium the current through the coil is linearly proportional to its angular deflection. This means that the galvanometer scales can always be linear, a great boon to the user.

  The D’Arsonsval meter at the left is designed for lecture-table use. A parallel beam of light from a small lamp reflects from the mirror attached to the top of the coil, and reflects back onto an external scale. It is from Denison University.

   At the right is a similar D'Arsonval meter at the University of Toronto. It was made by Ducretet of Paris.

   The base of this Queen D'Arsonval galvanometer is slightly different from the example above at the University of Toronto.

   This one is at Westminster College in western Pennsylvania.

   The small, self-contained D'Arsonval galvanometer at the right is in the collection of Richard J. Zitto.

   The coil of wire with red insulation can be seen in the upright, glass-fronted metal box. On either side are the poles of the permanent magnet used to set up the constant magnetic field used to provide the torque on the current-carrying coil. The coil is suspended from the top of the box by a thin phosphor-bronze strip, with the lower suspension being a loose coil of the same material.

   The reverse side of the curved strip mounted at the end of the long foot carries a scale with reverse-printed numbers. The user looks through the rectangular aperture mounted above the curved strip, and sees the scale in the small mirror attached to the coil. To keep the eye fixed in the same position, the image of the center line reflected from a small mirror on the surface of the glass cover is always kept in coincidence with the center line observed in the rotating mirror. 

   It is listed in the 1928 catalogue of the W.M. Welch Scientific Company at $9.00.

   There is a similar piece of Welch apparatus in the Greenslade Collection .

   The D'Arsonval galvanometer at the left is in the Jack Judson Collection at the Magic Lantern Museum in San Antonio, Texas. 

   Although it is unmarked, a look at the 1912 catalogue of the C.H. Stoelting Co of Chicago shows that it appears to be identical to the student's galvanometer sold for $6.75. For $15.00 it could be obtained in a wall-mounted form with a telescope and scale. The [broken] upper suspension, the coil and the pole pieces can be clearly seen.

   Self-contained galvanometers, such as the one by Rubicon at the right, used to be standard equipment for voltage measurements with a potentiometer. A 6 volt light bulb is used as a optical source, and, after a parallel beam is formed with a stop and a lens, the beam reflects from the mirror on the galvanometer movement, and, after several reflections inside to lengthen the optical lever, falls on the frosted-glass scale. Models with sensitivities as much as 2.5 nanoamperes per millimeter of scale division could be obtained for about $90 about 1950. 

   This example was contributed to the Greenslade Collection by Daniel Chaucer. 

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