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Stefan's equations of electrodynamics

The relation of the Ampere form, d2F12=( mu 0/4 pi )I1I2(3(dr1.r12/r12)(dr2.r12/r12)-2(dr1.dr2))r12/r123, to the Biot-Savart, or Grassmann form, d2F12=( mu 0/4 pi )I1I2dr2 V-product (dr1 V-product r12)/r123, of the law for the force between current elements is revisited from time to time. Recently experiments were reported to support the claim that the two forms are not equivalent. One hundred and twenty years ago J. Stefan (1869) answered most of the questions which reappear in contemporary work. Though the paper is seldom quoted it is instructive even today.