  We report on radio observations of the M8.5 dwarf LSR J1835+3259 and the L3.5
dwarf 2MASS J00361617+1821104, which provide the strongest evidence to date
that the electron cyclotron maser instability is the dominant mechanism
producing radio emission in the magnetospheres of ultracool dwarfs. As has
previously been reported for the M9 dwarf TVLM 513-46546, periodic pulses of
100% circularly polarized, coherent radio emission are detected from both
dwarfs with periods of 2.84 +/- 0.01 and 3.08 +/- 0.05 hours respectively for
LSR J1835+3259 and 2MASS J00361617+1821104. Importantly, periodic unpolarized
radio emission is also detected from 2MASS J00361617+1821104, and brightness
temperature limitations rule out gyrosynchrotron radiation as a source of this
radio emission. The unpolarized emission from this and other ultracool dwarfs
is also attributed to electron cyclotron maser emission, which has become
depolarized on traversing the ultracool dwarf magnetosphere, possibly due to
propagations effects such as scattering. Based on available v sin i data in the
literature and rotation periods derived from the periodic radio data for the
three confirmed sources of electron cyclotron maser emission, TVLM 513-46546,
LSR J1835+3259 and 2MASS J00361617+1821104, we determine that the rotation axes
of all three dwarfs are close to perpendicular to our line of sight. This
suggests a possible geometrical selection effect due to the inherent
directivity of electron cyclotron maser emission, that may account for the
previously reported relationship between radio activity and v sin i observed
for ultracool dwarfs. We also determine the radius of the dwarf LSR J1835+3259
to be > 0.117 +/- 0.012 R_Sol. (abridged)
