It is known that in mercury-vapour rectifier circuits certain important advantages may be obtained by the addition of a smoothing filter in which the first element is a choke. In this paper it is shown that the inductance of this choke has to be sufficiently great to ensure that at every instant at least one of the rectifying valves is passing current. A method of calculating the minimum value of inductance required, and some notes on the practical design of a choke, are given.