According to the MFJ manual, the center of a 2.1 mm. jack is 
positive.  Hope this helps.
PatAt 08:45 AM 9/3/2013, you wrote:
>Tom,
>
>The noise bridge comes with a short DC power cord with bare wire ends.  The
>cable is approximately 3 feet in length. for connecting to your station's DC
>power supply.  There is a 9 volt battery snap-on type connector for this
>battery inside the unit.  I will try to determine the polarity of the
>external DC connector and get back to you.
>
>73
>
>Howard #3
>
>----- Original Message -----
>From: "Tom Behler" <[log in to unmask]>
>To: <[log in to unmask]>
>Sent: Tuesday, September 03, 2013 8:34 AM
>Subject: MFJ Noise Bridge: Another Question
>
>
> > Hi, all.
> >
> > I have another question about the MFJ Noise Bridge we have been
> > discussing.
> >
> > I checked the MFJ web site, and found the MFJ 212 listed at a cost of $99,
> > which is what I expected.
> >
> > They indicate that the device can be run either via a direct DC
> > connection,
> > or a wallwart power supply, which costs another $14.
> >
> > Rather than spending the $14 on the extra wallwart, I'd rather run the
> > unit
> > off my rig runner via an Anderson Power Pole connection.
> >
> > However, I'm not sure how the plug for the DC connection should be wired.
> > For example, is the tip positive?
> >
> > In general, I'd be interested in how others on the list who have the noise
> > bridge power the unit.
> >
> > Tom Behler: KB8TYJ