  Series arrays of Josephson junctions show evidence of a mode in which all the
junctions oscillate in synchronism on voltage resonances appearing, in zero
external magnetic field, at multiples of the fundamental Fiske step spacing.
The measurements show that the current amplitude of the resonances increases
linearly as their voltages are summed. Investigation of the nature of the
coherent mode by magnetic field responses of arrays and isolated juctions
reveals that the oscillations take place in a parameter plane region where dc
magnetic fields only activate boundary current and flux-quanta dynamics can
take place.
