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by Bill Cardoso, CREATIVE ELECTRON "If the hand is held between the discharge tube and the screen, the darker shadow of the bones is seen within the slightly dark shadow- image of the hand itself... For brevity's sake, I shall use the expression 'rays'; to distinguish them om others of this name, I shall call them 'X-rays.'" —Wilhelm Conrad Roentgen, Munich, 1895 Introduction: A Brief History of X-rays It was the late part of the 19th century, and scientists around the world were fascinated by a new device called the Crookes tube (Figure 2). First shown to the world in 1878 by English scientist Sir William Crookes, the device's mysterious properties intrigued the best minds. One of these scientists was J.J. ompson, who, in 1897, figured out that the cathode rays inside the tube were negatively charged, which led to the discovery of the electron. He also figured out that a magnetic field could steer this stream of electrons, and with that, he paved the way for the invention of the television. As you can tell, a lot happened in the 1890s. For the context of this book, we'll focus on another scientist playing with Crookes tubes near Munich, Germany. Wilhelm Roentgen was curious why a fluorescent screen on the other side of his lab was glowing when the tube was on. He shielded the tube, and the glowing kept happening. He also noticed that the photographic paper in his lab was devel- oped with weird shadows. His "Eureka!" moment was realizing that his Crookes tube was generating new rays that could go through matter. Not knowing what to call it, he called it "X"—as in the unknown variable of an equation. And that's how "X-rays" were discovered. He promptly volunteered his wife to place her hand between the tube and some photo- graphic film, and the first X-ray was created (as seen in Figure 1). To Roentgen's credit, he decided not to patent his findings. He recognized that X-rays were such an impor- Book Excerpt: The Printed Circuit Assembler's Guide to... X-ray Inspection 18 SMT007 MAGAZINE I APRIL 2025 Figure 1: Photograph of X-ray image of Anna Ludwig, the wife of Wilhelm Roentgen, showing her hand. (Source: Gary Korkala)