Coils calibrated to accuracy of 0.2%. Coils have non-inductive windings so the box is applicable for both DC and AC experiments. All coils are firmly locked into place to eliminate physical strain on the wires. Other features: silkscreened cover, insulated binding posts, pointer knobs, and multiple switches with wiping contacts. Instructions provided. (5 1/2" x 7" x 2 1/2").