" Electromagnetic radiation predicted by Maxwell was produced experimentally by Hertz and he conducted experiments which showed that they travelled with a finite velocity. A value was first obtained by Blondlot. "
Selected frequencies were transmitted along a pair of parallel wires and reflected at the far end. This created a system of stationary waves with nodes and antinodes spaced a regular intervals. Knowing the frequencies and the distances between nodes, the speed of the radiation could be determined.
" Thirteen different frequencies between 10x106 and 30x106 Hz were employed, the total spread of the values of velocity was 5% and the average value was 297,600 km/sec. Blondlot concluded that the value agreed with the velocity of light and with the ratio of e.s.u and e.m.u. units within the accuracy of the experiment. "
(source: Froome and Essen, "The Velocity of Light and Radio Waves", Academic Press, 1969)
Today Blondlot is mostly remembered for his unfortunate "discovery", in 1903, of N-Rays. N-Rays enjoyed a short popularity but were ultimately shown not to exist.
The parallel wires used by Blondlot are known as Lecher wires. See, for example, this lecher demo (local copy).