We demonstrate that interior voltage probes may be used as a class of partially coupled contacts, able to feed or preferentially detect the innermost edge channel, as opposed to quantum point contacts or gated structures, which are simultaneously coupled to the N outermost edge channels. This unique feature of interior contacts has allowed us to measure the potential of the innermost edge channel directly. We find that a significant potential drop occurs between Hall plateaus due to the backscattering in the bulk of the sample, as predicted in a previous report. The temperature dependence of this effect is discussed. The data confirm the low interchannel scattering rate between the innermost and the N-1 other spin-degenerate edge channels. Finally, it is found that interior contacts used as current injectors preferentially populate the innermost channel, thus enhancing the Shubnikovde Haas oscillations. © 1991 The American Physical Society.