  Recent results based on the analysis of radio galaxies and their hot X-ray
emitting atmospheres suggest that non-radiating particles dominate the energy
budget in the lobes of FRI radio galaxies, in some cases by a factor of more
than 1000, while radiating particles dominate the energy budget in FRII radio
galaxy lobes. This implies a significant difference in the radiative efficiency
of the two morphological classes. To test this hypothesis, we have measured the
kinetic energy flux for a sample of 3C FRII radio sources using a new method
based on the observed parameters of the jet terminal hotspots, and compared the
resulting Q(jet) - L(radio) relation to that obtained for FRI radio galaxies
based on X-ray cavity measurements. Contrary to expectations, we find
approximate agreement between the Q(jet) - L(radio) relations determined
separately for FRI and FRII radio galaxies. This result is ostensibly difficult
to reconcile with the emerging scenario in which the lobes of FRI and FRII
radio galaxies have vastly different energy budgets. However, a combination of
lower density environment, spectral ageing and strong shocks driven by powerful
FRII radio galaxies may reduce the radiative efficiency of these objects
relative to FRIs and couteract, to some extent, the higher radiative efficiency
expected to arise due to the lower fraction of energy in non-radiating
particles. An unexpected corollary is that extrapolating the Q(jet) - L(radio)
relation determined for low power FRI radio galaxies provides a reasonable
approximation for high power sources, despite their apparently different lobe
compositions.
