We present the results of a new survey of 23 molecular clouds for the Zeeman effect in OH undertaken with the ATNF Parkes 64-m radio telescope and the NRAO Green Bank 43-m radio telescope .
The Zeeman effect was clearly detected in the cloud associated with the H ii region RCW 38 , with a field strength of 38 \pm 3 \mu G , and possibly detected in a cloud associated with the H ii region RCW 57 , with a field strength of -203 \pm 24 \mu G. The remaining 21 measurements give formal upper limits to the magnetic field strength , with typical 1 \sigma sensitivities < 20 \mu G .

For 22 of the molecular clouds we are also able to determine the column density of the gas in which we have made a sensitive search for the Zeeman effect .
We combine these results with previous Zeeman studies of 29 molecular clouds , most of which were compiled by Crutcher ( 1999 ) , for a comparsion of theoretical models with the data .

This comparison implies that if the clouds can be modeled as initially spherical with uniform magnetic fields and densities that evolve to their final equilibrium state assuming flux-freezing then the typical cloud is magnetically supercritical , as was found by Crutcher ( 1999 ) .
If the clouds can be modeled as highly flattened sheets threaded by uniform perpendicular fields , then the typical cloud is approximately magnetically critical , in agreement with Shu et al .
( 1999 ) , but only if the true values of the field for the non-detections are close to the 3 \sigma upper limits .
If instead these values are significantly lower ( for example , similar to the 1 \sigma limits ) , then the typical cloud is generally magnetically supercritical .

When all observations of the Zeeman effect are considered , the single-dish detection rate of the OH Zeeman effect is relatively low .
This result may be due to low mean field strengths , but a more realistic explanation may be significant field structure within the beam .
As an example , for clouds associated with H ii regions the molecular gas and magnetic field may be swept up into a thin shell , which results in a non-uniform field geometry and measurements of the beam-averaged field strength which are significantly lower than the true values .
This effect makes it more difficult to distinguish magnetically subcritical and supercritical clouds .