A serious defect of the galvanoscope and other simple galvanometers is that the needle responds to the vector sum of the horizontal component of the magnetic field of the earth and the magnetic field produced by the current being measured. As long as the galvanometer is being used for qualitative measurements, it is sufficient to turn the apparatus so that the compass needle points toward magnetic north when no current is applied to the coil.
The development of the astatic magnetic needle by Leopoldo Nobili (1784-1835) in 1825 took the opposite path by eliminating the effect of the earth's magnetic field on the needle. A pair of needles is mounted parallel to each other, but with the poles reversed. This combination has a net magnetic dipole moment of zero and thus has no preferred direction in the earth's magnetic field. The lower of the magnetic needles is inside the coil which carries the current under test, and alone experiences a torque due to the resulting magnetic field.
The suspension strand holding the pair of astatic needles
is missing from most of the astatic galvanometers shown on this page. The
suspension was probably a single strand of silk, which has no twist. The
current-carrying coils can be seen underneath the cards on which a graduated
circle is printed. However, the instrument cannot be calibrated from first
principles, and can therefore be used only to indicate the presence and direction
of a current.
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| I have set the astatic galvanometer at the right off by
itself because it is unusually well preserved. It is one of a group of pieces
of apparatus bought by the College of Wooster in Wooster, Ohio ca. 1900 to
replace equipment lost in a fire.
The galvanometer was made by Max Kohl of Chemnitz in Germany, and imported by the firm of James G. Biddle of Philadelphia. In the contemporary Kohl catalogue, it is listed as a demonstration galvanometer and priced at 65 marks ($15.00). The demonstrations must have been made to a relatively small number of students, as the position of the upper needle on the raised scale cannot be seen except at close range. Other galvanometers in this series had mirrors mounted just above the upper needle to serve as part of an optical lever, used to make the deflection visible to a larger number of students. |
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The Garland Collection of Classic Physics Apparatus at Vanderbilt University has a large collection of galvanometers from the period 1875-1900. Here are three of them, with accompanying quotes from Robert T. Lagemann's The Garland Collection of Classic Physics Apparatus at Vanderbilt University (Folio Publishers, Nashville, 1983):
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Left: "Some 28 cm tall, supported on three adjustable
brass legs (one missing), this early galvanometer has a rounded-end glass
cylinder to protect the moving parts against air currents. The dial for registering
the deflection of the needle is marked every degree and labeled every ten
degrees from 90 to 0 to 90. Marked "Ruhmkorff / à Paris"." Right: "The working parts are enclosed in a protective
cylindrical glass case 22.0 cm tall and 15.6 cm in diameter. It stands on
three adjustable brass legs. The dial for indicating deflections of the needle
is marked off in degrees and labeled every ten degrees from 90 to 0 to 90.
Marked Ruhmkorff / à Paris"."
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Left: "The ivory case surrounding the coil bears the words.
"Th. & A. DUBOSCQ / A PARIS / QUEEN & Co. SOLE AGENTS / in PHIL."
The galvanometer is somewhat unusual in that the electrical portion can be
lifted off the heavier three-legged base and worked on separately. The height
is about 23 cm." Right: This neat small galvanometer was used by high
school students in the Youngstown, Ohio area, and is now in the collection
of Richard Zitto.
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From the 1916 catalogue of the L.E. Knott Apparatus Company
of Boston:
"Astatic Galvanometer. This instrument was designed according to the requirements of the National Physics Course, built on the close-coil type, with astatic system in which one needle moves between the coils and one above. Aluminum dial 3½ inches, windings of No. 18 copper wire, resistance approximately one ohm. The instrument is provided with three leveling screws and binding posts. A convenient and sensitive instrument where low voltage is to be used ... $3.50" This instrument is at the physics department of the State University of New York at Fredonia. |
| The astatic galvanometer at the right is marked with the
name of the Chicago Laboratory Supply and Scale Company. About 1900 this
company was the sole survivor of four Chicago apparatus companies, including
Stoleting, but by 1912 the name had reverted to that of C.H. Stoelting Co.
This instrument is listed at $5.75 in the 1912 Stoelting catalogue, and is the lesser of the two astatic galvanometers in the catalogue. It is in the Jack Judson Collection at the Magic Lantern Collection in San Antonio, Texas. |
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The astatic galvanometer at the left is in the collection of Case Western Reserve University in Cleveland, Ohio, It was sold by James W. Queen of Philadelphia, is is very much like the example, above, from Wittenberg University. However, it has the more conventional bell-jar type glass cover. |