Stimulus Isolation - Why does this work?

[Matt Crema]

Hi,

I recently started using some DC stimulus isolators and I have a
question about how they work.

Each stimulus isolator is powered by its own 18V battery. Each is just
an NPN current source which can be used to sink DC pulses of current
through a load.

If I hook up two stimulus isolators to the same load, but reverse the
polarity of one of these, I can cause current to flow through the load
in either direction.

See http://www.bu.edu/ndl/Stimisolator.emf for a diagram of this circuit.

When Q1 is on and Q2 is off current flows in one direction.
When Q2 is on and Q1 is off current flows in the other direction.

My question is what happens when both Q1 and Q2 are off? If the
batteries are slightly different voltages then there is a voltage drop
across the load; why doesn't any current flow through the load? I
checked this case out and indeed no current flows through the load.

Is it because of the isolation of the two "grounds"? Is it because
there is no closed loop for current to flow? Actually I might have just
answered my own question. Still, I would appreciate any insight on this.

Thanks.
-Matt