Simplified approximate formulæ are given for calculating the attenuation, wavelength constant and characteristic impedance from the “open” and “closed” impedances. The limits of error of these expressions are ascertained, and the formulæ are developed directly in terms of the bridge readings. The limitations of the “open and closed” method on long lines are discussed, and the sensitivity of the bridge necessary to obtain a given accuracy in the measurement of attenuation is found.