Post Office Box Post Office Box Post Office Box – Physics Laboratory Equipment Post office box is education quality & Standard quality. Brass block, plug type, having 3 pairs of ratio coils, 10, 100, 1000 ohms and 16 series coil constant wire. All plugs are interchangeable with accuracy 0.1% Constant coil standard teak box ½” 3 ratio. And with managing coils ½” blocks. Most superior brass block 9/16” manganian coils. Available in coils – With 3 ratio coils. With 4 ratio coils
