Slaby rod for measuring radio frequencies

The Slaby rod, a variable-length coil wound on plastic tubing, was the first instrument to measure the length of electromagnetic waves. The strength of an induced field was shown on fluorescent paper, and the maximum field in relation to the length of the coil showed the wavelength. The rods are calibrated and can display different wavelengths depending on the diameter and coil winding. It remained a common instrument for measuring waves until 1905.