Earth Inductor


Earth Inductor


Physics; Electromagnetism


Physical Description

A rectangular, lengthwise symmetric, wood frame (163cm X 43.5cm) that houses two rings (inner diam. 23cm, outer diam. 30.5cm), one on each side of the line of symmetry, which acts as spindles for wire. The frame is held together with brass fittings and brass screws. A newer-looking aluminum crank centered on one long side of the frame is connected by a shaft to a set of seven numbered brass gears (diam. 8cm) along the opposite side of the frame. The gears rotate the spindles 360° while keeping them parallel. Each spindle, around which wire can be coiled over 100 times, has a commutator through which an electric current can be measured.

Functional Description

The instrument demonstrates Faraday’s Law of Induction, which states that a changing magnetic field (in direction or in magnitude) passing through a circuit generates an electromotive force, or voltage, in the circuit. The Earth Inductor instrument uses earth’s magnetic field, which is approximately constant in time but varies by location, to induce a current in a coil of wire. The operator turns the handle, rotating the wooden circle containing the coils of wire, which causes the amount of magnetic flux passing through the coil to change based on its angle with respect to earth’s magnetic field. An ammeter connected to the metal terminals records the generated electric field through time, and the angle of earth’s magnetic field lines can then be determined based on the angle of the coil at which the maximum electric field is measured. The large number of turns of wire in the coil, over 1000 turns, amplifies the resulting current because the observed electric field is proportional to the number of wire turns. Earth’s magnetic field is relatively weak, normally requiring very sensitive instruments to measure. Mutual inductance is a second phenomenon demonstrated by the earth inductor, and is the reason for having two connected coils. The current generated by a single coil from earth’s field is not constant with time, and therefore generates its own independent magnetic field. This secondary field influences the electric field of a secondary, distant coil, and the difference between the primary induction and the secondary induction can be measured.


Ajay Vasu, Zane Barker, Daniel Ratkos, Luke Weidner, Lindsey Wells






physical object


United States

Physical Dimensions

Outer Frame: 163 x 43.5 x 9 cm

Outer Diameter of coil: 30.5cm
Inner Diameter of coil: 23cm
Thickness of coil: 4cm
Corner of frame to attachment point of coil: 20cm

Weight: 37lbs (approx)

Gear Diameters: 8cm
Number of gears: 7
Total width (width rotator and gear rotor): 63cm
Pitch of the gear: 4mm


Wood, Brass, Copper, Aluminium, Plastic


Charles Edouard Joseph Delezenne



History of the Object

History at MTU: Dean Fisher, for which Fisher Hall is named, was a professor of Physics in the 1920s-1940s. The exact history of the device is unknown, but it’s reasonable to assume due to the apparent age of the device as well as Fisher’s research interests, that Dean Fisher may have made use of it as a teaching device in the first Geophysics classes that were added to the school’s catalog in the 1930s. The course catalogs from that time period list course B110, or Geophysics, specifically mention that “magnetic, gravitational, elastic, thermic, and radio-activity methods [of prospecting] will be studied.” The Earth Inductor could be one of the pieces of prospecting equipment used in the four hour a week field portion of the class, drawing on Dean Fisher’s famous use of earth resistors to determine approximate mineral composition of the earth below. [2] History before MTU: This instrument appears to have been custom-made for use by MTU faculty in order to demonstrate mutual inductance [3]. The build of the device is very similar to two stacked “Earth Inductors”, with a gear system forcing coupled motion. Research into similar dual-coil models has come up short, as this instrument was most likely made uniquely for use at the university. The earth inductor, or Delezenne’s Circle, was first developed by Charles Edouard Joseph Delezenne in the 1840’s, and documented in his paper, “Notions élémentaire sur les phénoménes d'induction”, published in the 1844 edition of Memoirs of the Society of Sciences, Agriculture and Arts of Lille (pp 1-132). Delezenne’s text was published in French only, but the title translates roughly to “Basic notions about induction phenomena.” Delezenne’s instrument was developed to be used as a tool for explaining the phenomenon of induction, discovered by Michael Faraday, to students. In the introduction to his text, he states, “It seemed to me, then, that young students, novices still in the study of the somewhat elevated parts of science, but wishing to learn the induction phenomena discovered by Mr. Faraday, needed to be prepared for fruitful reading.” (Original French: “Il m'a donc semblé que de jeunes étudiants, novices encore dans l'étude des parties un peu élevées de la science, mais désireux de s'initier aux phénomènes d'induction découverts par M. Faraday, avaient besoin d'être préparés pour lire avec fruit. [1]”)


Michigan Tech Physics Department


[1]Delezenne, C. E. (1844). Notions élémentaire sur les phénoménes d'induction. Mémoires de la Société des sciences, de l'agriculture et des arts de Lille,23, 1-132. Retrieved March 1, 2017, from

[2] Course Catalog for Fall 1937. Houghton, MI: Michigan College of Mining and Technology, 1937

[3] Young, Charles F. Interview by Luke Weidner. Personal interview. Michigan Tech, February 16, 2017.





Ajay Vasu, Zane Barker, Daniel Ratkos, Luke Weidner, Lindsey Wells, “Earth Inductor,” Michigan Tech Inventory of Historic Scientific Instruments, accessed March 4, 2021,

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