psd of an ac voltage signal In an lab class experiment who's subject of interest was Johnson's noise I measured the ac voltage of a battery. Now I wish to find the psd of the measurement and show that it is constant.

1.Is such a thing even possible with 255 ac measurements in a $0.043s$ spacing?


How can it be done without a direct measurement of the voltage?
To compute the PSD, you'd need full amplitude/phase measurements of the noise signal. 

If by "255 ac measurements", you mean measurements of the RMS amplitude by a digital multimeter, then you do not have the necessary phase information. 

If by "255 ac measurements", you mean you are using an oscilloscope that is AC coupled, then you can consider the measurements to be a measurement of the voltage with the average value ("DC component") removed.   If we consider the voltage signal to be
$$
v_\text{total}(t) = v_\text{DC} + v_\text{AC}(t),
$$
$v_\text{DC}$ is going to be nearly entirely the "normal" contribution of the battery, and constant with time, and $v_\text{AC}(t)$ is going to vary with time and due entirely to thermal noise.  If your measurement is measuring $v_\text{AC}(t)$, then that is what you want anyways.

Once you have waveform samples, the PSD can be estimated using the periodogram (easiest, but noisy) or a more advanced method like Welch's or Blackman-Tukey method.