The ETB-260 Post Office Box Trainer is a compact and educational instrument designed to verify and measure unknown resistance using the Wheatstone Bridge principle. Commonly used in physics and electronics labs, this trainer includes precision resistors and terminals arranged to replicate the traditional Post Office Box setup. It provides students and trainees with hands-on experience in resistance measurement, circuit balancing, and bridge network analysis. The trainer is ideal for foundational experiments in basic electrical engineering and applied physics.