This is a chart showing the non-inverting input voltage (mV) versus the output from the op amp, as read by the Arduino (0-1023). With the correct gain multiplier the op amp does vary the output as the input voltage changes. However, it's extremely non-linear. The configuration that created these data was a NTE889M with 26x gain. So, I need to find the right op amp to provide a smooth, straight output voltage, but it seems like all of the variable voltage amplifies were designed for audio/video applications and expect dual power supply connections.