Beginning in the early 1930s General Electric and Westinghouse marketed a new type of Christmas lamp known as the Detector. The new Mazda Detectors contained a shot of neon gas that would ionize when the tungsten filament failed. A small unpainted “window” was left near the bottom of the glass envelope. When the lamp’s filament failed the window provided a means for seeing the neon glow. The glow indicated the lamp had failed, thus locating the dud lamp was simplified. Remember, in more traditional series burning light strings, when one lamp fails the entire string goes dark. The Mazda Detector was a clever attempt to solve the problem of finding the burned-out lamp on the string.