USB hub boost input voltage

What is that circular socket on USB hubs for? A power boost? By USB
hubs I mean the ones that turn one USB socket into four, etc.

I have two USB hubs, an Alcor Micro Hub, and a NEC HighSpeed Hub. Each
has a little round socket presumably for DC input. But no mention is
made about polarity and voltage, nor when one would need to give an
extra power boost.

There must be some standard. I suppose I can just connect the right
voltage wallwart to it? Wait, none of the standard adapters seem to
fit it. I suppose this is to prevent something stupid from occurring?

 

The circular socket you can see is to give the usb hub a current boost.
A standard usb port can supply 500ma at 5V But if you want to run a
couple of external hard drives and a webcam you will be drawing an amp
or more and it won't work. This is where you plug your wall wart in and
everything works again.

 

The units would normally spec, or come with, an appropriate wallwart in
most circumstances. You might try looking them up. Generally speaking,
the outer connector is V-, the inner V+ (but as this varies, look them
up first).

The reason for the external power is possibly not only to provide extra
power beyond the USB spec, as the devices may be specified as
'self-powered' and would therefore require the wallwart to operate as
advertised.

Cheers

PeteS