The document discusses the cathode ray oscilloscope (CRO), which is an electronic test instrument used to observe changing electrical signals over time. It describes the key components of a CRO including the cathode ray tube, vertical/horizontal controllers, triggers, and displays. The document explains how a CRO works by amplifying input signals and using electron beams to produce waveforms on the screen. Various sweep modes, synchronization methods, and applications of CROs for measuring voltage, current, and examining waveforms are also covered.