How was Dorsey able to measure speed of light with 3 significant digits in the year of 1907?
They just had to measure the capacitance of a capacitor!
If you can build a precise dimensioned capacitor you can work out it's capacitance with an equation including the vacuum permittivity ($ε_0$). It's easy to build the dimensions of the metal plates in a capacitor to high accuracy, and you can measure current accurately so by measuring the capacitance you get an experimental value for this constant.
Then you get a value of $c$ from $c^2 = 1/(ε_0μ_0)$