“…The first kind is linear and the second quadratic with respect to the Weyl tensor, but a basis for both of them is the Maxwell field. Conserved quantities which are linear with respect to the CYK tensor were investigated many times (cf [1,21,22,25,26,28,35,36]). On the other hand, quadratic charges are less known and have usually been examined in terms of the Bel-Robinson tensor (see e.g.…”