Two and a half weeks later, effective 1 January 1939, the Germans increased the number of plug connections to 7–10, which, writes Rejewski, "to a great degree, decreased the usefulness of the bombs." Zygalski's perforated ("Zygalski") sheets, writes Rejewski, "like the card-catalog method, was independent of the number of plug connections. But the manufacture of these sheets, [...] in our [...] circumstances, was very time-consuming, so that by 15 December 1938, only one-third of the whole job had been done. Germans' [introduction of rotors IV and V [...] increased the labor of making the sheets tenfold 60, or ten times as many, sets of sheets were now needed, considerably exceeding our [...] capacities."