| The Earth Inductor is a flip coil used to measure the
magnitude and direction of the earth's magnetic field. The rotating coil
is quickly flipped through 180 degrees, and the output is fed to a ballistic
galvanometer. This device, with a long period of oscillation, measures the
total charge delivered by the induced EMF; the charge is then proportional
to the magnitude of the magnetic field threading the coil.
The experiment is done with the coil horizontal (to find the vertical component of the earth's field) and vertical (to find the horizontal component). The direction and net field can then be readily calculated. The apparatus was devised by Charles Edouard Joseph Delzenne (1776-1866) and described in his two papers "Notices elementarie sur les phenomenes de induction", Lille, Memoires de la Societe des Sciences, 23, 1-132 (1844); 27, 10-160 (1847) |
|
|
|
| The earth inductor at the right was made by Phillip Harris, and in the Spring of 2000 was at James Kennedy Antiques in Durham, North Carolina. | 
|
|
The earth inductor at the left in in the Garland Collection
of Classical Physics Apparatus at Vanderbilt University. There is no maker's
mark. The circular coil has an outer diameter of 27.5 cm.
The construction involves no iron, only wood and brass, to eliminate spurious magnetic effects. |
| The Earth Inductor at the right was made by Gaertner of Chicago, and is in the collection of Denison University in Granville, Ohio. | 
|
|
The Gaertner Earth Inductor at the left is in the Greenslade Collection. |
The Earth Inductor at the right was made by W.G. Pye & Co. of Cambridge, England. It is described in the 1911 catalogue as: "Earth Inductor or Delzenne's Circle, coil frame and stand of polished mahogany, divided dial for shewing the angle of the axis of the rotating coil. Complete with commutator ...£3 . 10 . 0 " The instrument is in the collection of Hobart and William Smith Colleges in Geneva, New York. |
|
|
This is another form of the earth inductor, sometimes
called Delzenne's Ring. It is mounted in a rotator and spun about its
vertical axis. A pair of spring-metal contact blades rub against the commutator
segments connected to the multiple-turn winding. If the rotation is slow,
a sensitive zero center galvanometer will deflect alternately right and left.
Today I would do the demonstration with an oscilloscope as a detector. The
ring is 30 cm across.
The apparatus is in the Greenslade Collection. |
This Earth Inductor is described in the 1920 catalogue of the Central Scientific Company of Chicago as: "EARTH INDUCTOR, Cenco improved form, has 1,000 turns of wire of about 250 ohms resistance, used wound on a non- magnetic metal ring 22 cm in diameter. On releasing the catch, a [flat] spring causes the coil to rotate through 180 degrees, when it is caught and held by a second catch. ... May be used in either vertical or horizontal plane ... $42.00" The example at the right is in the Greenslade Collection, and has serial number 55. The plate visible on the front corner notes that the coil resistance is actually 255 ohms, with an inner diamter of 217 mm and an outer diamter of 238 mm. |
|
|
This small pieces of apparatus, made by James W. Queen
of Philadelphia, is in the collection of Franklin and Marshall College
in Lancaster, Pennsylvania.
It is a coil of wire that can be flipped rapidly about the axis of the shaft running through it. Flexible connections are made to the ends of the wire. I have not found this in a 19th century Queen catalogue, but it certainly must be a portable flip coil for sampling the local value of the magnetic field. If so, it is then used in the same fashion as the large instruments illustrated above. |
This piece of apparatus is at Case Western Reserve University in Cleveland. It was made by Hartmann & Braun of Berlin, and is quite small, measuring only 35 cm along the front of its base. I have included it with the earth inductors, but it may well be a varaible mutual inductance. |
|