Some welding electrodes will run on AC (alternating current). That is - the electrons shunt backwards and forwards to transfer electrical energy, like the Mains does.
All "6013's" run very very smoothly on AC
Some electrodes will only run on DC (direct current). That is, the current always flows in the same direction. AC rods will run on DC, but not vice-versa. If you know your welding and know what rods will give what effect, you will sometimes choose rods which will only run on DC. The familiar DC-only rods are "6010's" and "7018's". Sorry - if you want more information, look up about these and welding rods in general on the Web.
Some welding machines have a built-in rectifier. In "the old days", rectifiers were sold separately, and you coupled them between the welding transformer and the work, interposed in the welding leads. Ones sold by the solid-reputation companies then were very well engineered and some survive in store-rooms, use unrecognised - as was the case here.
R. Smith, 19 January 2008