Experiments on chaotically oscillating arrays of nickel electrodes in sulfu
ric acid were carried out. A global periodic forcing is added to a base sta
te of 64 weakly coupled elements by means of variations in the applied pote
ntial. Intermittent, unstable chaotic clusters are observed at low forcing
amplitudes. Stable clusters, first chaotic and then periodic, form as the f
orcing amplitude is increased. Stable cluster states consisting of two, thr
ee, and four different clusters are observed at forcing amplitudes below th
at at which synchronization occurs.