It is easy to see or hear the effects of electrical devices. Hearing aids amplify sound, lightbulbs light, and cellular telephones connect you to the world with pictures and sounds. However, the inner workings of electrical circuits cannot be directly observed. Specialized instruments must be used to detect and measure electrical quantities within circuits. The discussion of these electrical quantities at first may seem somewhat abstract since you cannot see or hear them. However, in time, as your experience with electrical instruments grows, you will become comfortable with identifying, measuring, and manipulating electrical quantities within circuits.