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We report on radio observations of the M8.5 dwarf LSR J1835+3259 and the L3.5dwarf 2MASS J00361617+1821104, which provide the strongest evidence to datethat the electron cyclotron maser instability is the dominant mechanismproducing radio emission in the magnetospheres of ultracool dwarfs. As haspreviously been reported for the M9 dwarf TVLM 513-46546, periodic pulses of100% circularly polarized, coherent radio emission are detected from bothdwarfs with periods of 2.84 +/- 0.01 and 3.08 +/- 0.05 hours respectively forLSR J1835+3259 and 2MASS J00361617+1821104. Importantly, periodic unpolarizedradio emission is also detected from 2MASS J00361617+1821104, and brightnesstemperature limitations rule out gyrosynchrotron radiation as a source of thisradio emission. The unpolarized emission from this and other ultracool dwarfsis also attributed to electron cyclotron maser emission, which has becomedepolarized on traversing the ultracool dwarf magnetosphere, possibly due topropagations effects such as scattering. Based on available v sin i data in theliterature and rotation periods derived from the periodic radio data for thethree confirmed sources of electron cyclotron maser emission, TVLM 513-46546,LSR J1835+3259 and 2MASS J00361617+1821104, we determine that the rotation axesof all three dwarfs are close to perpendicular to our line of sight. Thissuggests a possible geometrical selection effect due to the inherentdirectivity of electron cyclotron maser emission, that may account for thepreviously reported relationship between radio activity and v sin i observedfor ultracool dwarfs. We also determine the radius of the dwarf LSR J1835+3259to be > 0.117 +/- 0.012 R_Sol. (abridged)

Confirmation of the Electron Cyclotron Maser Instability as the Dominant Source of Radio Emission from Very Low Mass Stars and Brown Dwarfs