In our EPR experiment, the signal looks like the "first derivative" part of the above picture. Why is this? What does the "first derivative" mean and why is it the quantity our instruments detect in such an experiment?
See http://www.bruker-biospin.com/cwpractice.html (Bruker know a thing or two about making spectrometers :-). I quote:
So the spectrometer measures the first derivative. To get the signal you'd need to integrate the output from the spectrometer, but normally you wouldn't do that as you can get the line position and width directly from the first derivative.