Instrumenting the controlled-potential techniques described in Chapter 3 is not difficult within certain limits of time and current. Nevertheless, beginners and old-timers alike often seem to have only the vaguest notion of how their instruments function and the logic implicit in various experimental designs. One reason for this is that there seems to be no good introduction to the subject. The literature is either too superficial or inordinately complex. Our goal here is the intermediate ground. We attempt to provide a phenomenological basis for several fundamental instruments on a level satisfactory for utilizing them intelligently. The steps up the ladder to more complex situations (e.g., very fast experiments, alternating-current experiments, two working electrodes at different potentials in the same solution, iR compensation, digital computer control and analysis, etc.) are relatively trivial ones. In our experience, most students encounter difficulties with electrochemical instrumentation at the outset primarily due to a lack of appreciation of basic electricity. The meaning of such terms as charge, voltage, current, and resistance must be intuitively clear.