The magnetic flux through an interstellar cloud or through a part of the cloud must decrease considerably by the drift of plasma and magnetic field in order that stars form in the cloud. Because most grains are negatively charged in dense clouds (Elmegreen 1979; Umebayashi and Nakano 1980), they retard magnetic flux leakage in addition to ions (Elmegreen 1979). We investigate this effect for different situations and obtain the following results (for details, see Nakano and Umebayashi 1980):

1. 1. For nearly spherical clouds of mass ≳ 103M⊙ sustained by magnetic force the friction of grains is efficient at nH⊙ ≳ 105 cm−3, and the magnetic flux leakage time tB is greater than a few million years at any density. Grains drift as fast as ions and electrons.

2. 2. For spherical clouds of smaller mass and disk-shaped clouds, tB becomes much smaller and the drift of grains is much slower than ions and electrons at nH ≳ 106 cm−3.

3. 3. Thus magnetic flux leakage occurs mainly in the condensations described in 2, and the abundance of heavy elements in stars deviates little from that of the parent clouds because of small grain drift.