In any laboratory for electronics and electrical engineering, a Cathode Ray Oscilloscope, or CRO for short, is today a fundamental, significant, and multipurpose tool. You had the chance to use a voltmeter and a multimeter to measure the voltages of an ac source and a dc source in the previous exercise. If you look at how these voltages change over time, you'll see that the ac voltage changes sinusoidally while the dc voltage stays constant (a voltage versus time graph shows a straight line parallel to the x-axis). Although we can learn the voltage magnitudes from an ac-voltmeter or multimeter, we are unable to learn the waveform characteristics of an ac or dc signal.