Beff = (1-s) Bo
where s describes the shielding of the effective magnetic field by the electrons surrounding the nucleus. The experiment described previously can be modified to detect nuclei resonating at different frequencies by making the transmitter sweep through the range of expected frequencies and recording the receiver response for each resonance. A machine built to such a specification is called a continuous wave or CW machine, such as one of the R24 machines in the teaching laboratories. (Technically, it is actually easier to adjust Bo than +w on some machines, but that is a minor detail). However, a CW machine has one major disadvantage, which is that only one resonant frequency can be measured at any instant. For protons, the detected frequencies can differ by 600 Hz or more (at 60 MHz) and the time constants for relaxation described above limit the detectable response rate of the protons. In effect, this means that one cannot sweep through this frequency range at a rate of more than about 1Hz per second, ie it would take 600 seconds to record the spectrum.