On the magnetic effect of electric convection
Type:    Journal, Article
Title:    On the magnetic effect of electric convection
Author(s):    Rowland, H. A.
Date:    1878
Abstract:    Maxwell had predicted that a moving charge would act like an electric current "but that the action exists has not yet been proved experimentally or theoretically". An annular part of an insulating disc of diameter 21 cm was made the central electrode of a symmetrical plane-parallel capacitor, and a voltage applied (through a slip ring) so as to induce a known charge on the disc surfaces. The disc was rotated at 61 rev/sec about a vertical axis. To measure any magnetic field resulting from the motion of the charge, an astatic magnetometer was used, having a magnet spacing of 18 cm, and with the lower magnet above the disc. The voltage was measuring by observing the spacing between two spheres at which a spark occurred. There were many sources of noise, but he claims that the results were reproducible in good conditions. The agreement between predicted and measured magnetic field was about 5%; this difference was reduced if he used Weber's value for c, the speed of light, rather than Maxwell's value. (The value of c enters the calculation because of the need to convert the induced charge density from esu to emu. Rowland points out that the experiment could be used to measure c.) This is the first experimental confirmation that a moving electrostatic charge behaves like a current; it was done in the laboratory of Professor Helmholtz in Berlin. A note says "The idea of the experiment first occurred to me in 1868 and was recorded in a note book of that date. In his 1879 paper Rowland says that at the time of this experiment he "suggested to Professor Helmholtz that a theory of the earth's magnetism might be based upon the experiment. But upon calculating the potential of the earth required to produce the effect, I found that it [4x1016 V] was entirely too great to exist without producing violent perturbations in the planetary movements, and other violent actions".
Journal (full):    American Journal of Science
Volume:    3
Issue:    15
Start Page:    30
End Page:    38
Link (full):    http://www.ajsonline.org/content/s3-15/85/30.extract

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