Electric potentials around the heart. What and How does the EKG measure? Recently I've been having trouble understanding how(and even what) the EKG measures and have not been able to find a satisfactory response. The health science forums haven't been much help and I thought since EKG is practically pure physics I might as well ask from the experts themselves. Keep in mind I have only basic rusty high school physics.
QUESTION:
Anyone who has seen an EKG will know that it commences with a P wave and once the P wave ends an isoelectric line follows(PR segment). Now everywhere I search I find the majority saying an EKG is a voltmeter/galvanometer (more familiar with a voltmeter but once I start trying to understand galvanometers electromagnetic fields induced by currents and stuff pop up and I just break down). Now my question is if an EKG truly is a voltmeter then why is it that when the atria are fully depolarized while the ventricles are polarized the EKG doesn't pick up this huge potential difference (and hence show an isoelectric line, If anything I'd expect the P wave to be at it's greatest peak then, since there is such a big difference in charges between the 2 areas of the heart).
If you feel like I need to understand some prerequisites to fully understand and you can't be bothered to explain them just share some useful resources or tell me what I gotta know to truly understand. Thanks for your help
 A: 
Now my question is if an EKG truly is a voltmeter then why is it that
  when the atria are fully depolarized while the ventricles are
  polarized the EKG doesn't pick up this huge potential difference..?

The term depolarization, in this context, is very specific and indicates the change of the cell membrane potential, during the first phase of the action potential, from negative (rest) potential to positive. It is followed by repolarization. These changes of the membrane potential send waves though the body, which can be picked up by EKG probes as voltage pulses. 
A positive P-wave represents depolarization of the atria. A positive QRS complex, which occurs a little later, represents depolarization of the ventricles.

So, the electrical activity of the heart is not due to a potential difference between different parts of the heart, e.g., atria and ventricles, but due to action potentials, spreading from atria down to ventricles and activating appropriate muscles at appropriate times.   
The voltage pulses picked up by the probes correspond to action potentials in various parts of the heart, rather than potential differences between various parts of the heart.
For details you can read about action potential.
