Instruments for measuring the polarization of the cosmic microwave background (CMB) must be designed for accuracy as well as precision. The requirement for precision translates into a need for detectors with unprecedented sensitivity. Accuracy requires good methods for calibrating the response of the instrument to small polarized signals superimposed on large unpolarized signals. Since well-characterized polarized astrophysical millimeter sources are in short supply, we present an alternative method here. A flat metal plate is mounted in front of the telescope and nutated about a vertical axis, providing a varying polarized signal of amplitude near 30 mK. .