The S/WAVES experiments on the two STEREO spacecraft measure waves, both in situ plasma waves and remotely generated waves such as Type II and Type III solar bursts. A part of the experiment is aimed at understanding the generation of electromagnetic waves from electrostatic Langmuir waves. For this, rapid measurements of plasma density, sufficiently rapid to be on the time scale of Langmuir wave fluctuations, are deemed necessary. Measurements of the potential of the antennas relative to the spacecraft can supply these rapid measurements. The antennas were not provided with a bias current, and so this unbiased technique has not been used previously. However, the cylindrical antennas of S/WAVES respond to temperature as well as the density of the ambient plasma, giving five quantities, ne, Te, and 3 components of E, to be determined from the three measurements of antenna potential. The work presented here discusses the analysis and interpretation of these measurements from the early part of the mission, when there were frequent observations of foreshock Langmuir waves to use for calibration. A model of the photoemission-plasma equilibrium has been constructed, using these and other measurements. It is shown that the response to one or a few of the five quantities may be negligible, depending on the phenomenon observed, so that useful measurements are obtained of the others. Application to observation and analysis of various plasma wave phenomena w111 be discussed.