Realization of Bullard’s disc dynamo

R. Avalos-Zúñiga, Janis Priede

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4 Citations (Scopus)
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We report experimental results from three successful runs of a Bullard-type homopolar disc dynamo. The set-up consisted of a copper disc with a radius of 30 cm and thickness of 3 cm which was placed co-axially beneath a flat, multi-arm spiral coil of the same size and connected to it electrically at the centre and along the circumference by sliding liquid-metal contacts. The magnetic field was measured using Hall probes which were fixed on the top face of the coil. We measured also the radial voltage drop across the coil. When the disc rotation rate reached Ω ≈ 7 Hz , the magnetic field increased steeply approaching B 0 ≈ 40 mT in the central part of the coil. This field was more than two orders of magnitude stronger than the background magnetic field. In the first two runs, the electromagnetic torque braking the disc in the dynamo regime exceeded the breakdown torque of the electric motor driving the disc. As a result, the motor stalled and the dynamo was interrupted. Stalling did not occur in the third run when the driving frequency was set higher and increased faster. We also propose an extended disc dynamo model which qualitatively reproduces the experimental results.
Original languageEnglish
Number of pages17
JournalProceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
Issue number2271
Early online date8 Mar 2023
Publication statusPublished - 29 Mar 2023

Bibliographical note

© The Authors. Published by the Royal Society under the terms of the
Creative Commons Attribution License
by/4.0/, which permits unrestricted use, provided the original author and
source are credited.


This work was supported by the National Council of Science and Technology of Mexico (CONACYT) through grant no. CB-168850 and the National Polytechnic Institute (IPN) through grants nos. SIP-20211736 and SIP-20220849. Acknowledgements Publisher Copyright: © 2023 The Author(s)


  • disc dynamo
  • geodynamo
  • homopolar generator

ASJC Scopus subject areas

  • Mathematics(all)
  • Engineering(all)
  • Physics and Astronomy(all)


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