This third Volume contains Weber's main works related to diamagnetism, including his Third major Memoir on Electrodynamic Measurements (1852). These works are followed by three papers published by Weber and Rudolf Kohlraush in 1855-1857 in which they presented the measurement of Weber's fundamental constant c appearing in his force law. Weber and Kohlrausch's 1857 work is the Fourth major Memoir on Electrodynamic Measurements. Soon after this measurement, Kirchhoff and Weber succeeded in deducing the complete telegraph equation from Weber's electrodynamics. Their works were published in 1857 and 1864. When the resistance of the wire was negligible, the telegraph equation reduced to the wave equation. The velocity of propagation of an electric wave along the wire was equal to the known light velocity in vacuum. This remarkable result of Weber's electrodynamics indicated for the first time in the history of physics a direct and quantitative connection between electrodynamics and optics. This volume contains the English translations of Kirchhoff's two papers of 1857, together with a paper by J. C. Poggendorff emphasizing the independent researches made by Weber and Kirchhoff on this subject in which both scientists arrived simultaneously at similar results. Weber's 1864 work is his Fifth major Memoir on Electrodynamic Measurements. The translation of this paper is also included in this Volume.