We constructed a heater block comprising a central circular column and a peripheral concentric cylinder separated by a 0.1-mm radial gap so that liquid inflow from the surroundings scarcely affects the central part. For more accessible temperature control in transition boiling, these two parts were connected at the base. Analyzing the stability of accessing transition boiling shows that the effective negative slope of the boiling curve can be reduced by 20%. We conducted boiling curve measurements and high-speed observations of liquid on the surface, and these reveal the following. The wall superheat of the central part of the heater is higher than that of the peripheral part in the case of low-heat-flux nucleate boiling, which results from the existence of large nucleation-free areas. The central part always enters the transition boiling regime before the peripheral part does so. We measured the boiling curves for two types of surface: those with perfect clearance and those with cutting-generated burrs forming thermal bridges near the surface. The critical heat flux on the central part increases when the thermal bridges exits. We observed the liquid left under coalesced bubbles. Near the point of critical heat flux, active nucleate boiling takes place. At higher superheat film-boiling-like behavior with intermittent bubble bursts was observed.