HESI Radiologic Technologist Practice Exam 2026 – Complete Study Guide

Thermionic emission from a heated filament provides the electron source that lets an X-ray tube work. William Coolidge refined this idea into a practical, reliable device by using a heated tungsten filament as the cathode inside a high-vacuum tube, paired with a target anode. This hot-cathode design allowed a controllable, high current of electrons to flow toward the target, producing X-rays efficiently and with greater tube life and stability. That combination—the heated filament emitting electrons, a controlled high-vacuum environment, and a readily targetable anode—became the standard configuration for modern X-ray tubes used in radiography. Roentgen’s Crookes tube relied on a cold cathode, Edison contributed to early vacuum tube improvements, and Tesla’s work was broader in electrical generation and other devices, but the modern X-ray tube design hinges on the hot-cathode approach perfected by Coolidge.