Canal rays are streams of positive ions that pass through tiny holes in the cathode of a discharge tube, the counterpart to cathode rays (electrons). Stark precisely measured how the wavelength of light emitted by these fast‐moving ions shifts with their speed, providing solid experimental proof of the Doppler effect. He also observed that applying an external electric field causes spectral lines to split in a regular pattern—a phenomenon later named the Stark effect. These findings imposed crucial experimental constraints on the emerging quantum theory and supported the idea of discrete atomic energy levels. Today, they underpin technologies such as laser spectroscopy and plasma diagnostics.

Stark analyzed the emission spectra of canal‐ray ions travelling at ≈1–3×10^5 m/s with a high‐resolution spectrograph, carefully eliminating spurious instrumental shifts. The observed wavelength displacement obeyed Δλ/λ = v/c, in perfect accord with the classical Doppler formula, providing a novel method to measure high ion velocities. He then applied electric fields of about 10^5 V/cm to hydrogen and other atomic spectra and recorded the equal-spaced splitting of n→n' transitions. This Stark effect, regarded as the electric analogue of the Zeeman effect, demonstrated that degenerate atomic energy levels are lifted by an external field. The work prompted refinements of the Bohr–Sommerfeld model and later became a cornerstone for perturbation theory in quantum mechanics.

Stark generated canal rays of positive ions (predominantly H⁺, H₂⁺, He⁺) in Hittorf tubes biased at 10–40 kV through perforated cathodes. In his 1912–1913 series he measured the Hα displacement to 10⁻³ Å precision, deriving ion velocity distributions via v ≈ (Δλ/λ)c. From 1913 onward he inserted a tunable parallel-plate capacitor between electrodes, applying E ≈ 0–2×10⁵ V/cm and photographing the multiplet splitting of Balmer and Paschen lines. He demonstrated that the primary splitting component scales linearly with field strength, providing the first observation of the linear Stark effect arising from the E1 perturbation H' = –eEz. These data were later employed to test Kramers–Heisenberg matrix elements and δ-function perturbative treatments, serving as benchmarks for transition-probability calculations in early quantum mechanics.