An option for deriving c that does not directly depend on a measurement of the propagation of electromagnetic waves is to use the relation between c and the vacuum permittivity ε 0 and vacuum permeability μ 0 established by Maxwell's theory: c = 1/(ε 0 μ 0). The vacuum permittivity may be determined by measuring the capacitance and dimensions of a capacitor, whereas the value of the vacuum permeability was historically fixed at exactly 4π × 10 H⋅m through the definition of the ampere. Rosa and Dorsey used this method in 1907 to find a value of 299 710 ± 22 km/s. Their method depended upon having a standard unit of electrical resistance, the "international ohm ", and so its accuracy was limited by how this standard was defined.