Townsend's early experimental apparatus consisted of planar parallel plates forming two sides of a chamber filled with a gas. A direct current high-voltage source was connected between the plates; the lower voltage plate being the cathode while the other was the anode. He forced the cathode to emit electrons using the photoelectric effect by irradiating it with X-rays, and he found that the current flowing through the chamber depended on the electric field between the plates. However, this current showed an exponential increase as the plate gaps became small, leading to the conclusion that the gas ions were multiplying as they moved between the plates due to the high electric field.