Re: What Defines Voltage Levels?

To answer your question, let's start with a couple of definitions.

Voltage is "potential energy per unit charge".  It is also commonly 
referred to as "potential difference".  If you had an arbitrary collection 
of discrete charges at points P_1, P_2, etc., you would find the voltage 
at a point, P_0, by dividing the amount of charge at each of the other 
points by the distance from each point to P_0, then summing all these up.  
In this equation, amount charge is in the numerator and distance is in the 
denominator.  You can create a large voltage either by having a lot of 
charge, or a little charge but packing it close together without allowing 
the charge to flow (no current).

As you mentioned in your question, current is the flow of electrons: the 
more electrons flowing past a point at a given time, the higher the 
current.  You can only have a high current if you have a large source of 
electrons.

In the case of a battery, electrons are freed up as a consequence of an 
electro-chemical reaction.  The voltage is a function of the chemistry 
used, the current is a function of reaction rate (and size).  There are 
lots of electrons available inside the battery, it becomes a question of 
how fast they can be made usable by the chemical reaction.

In the case of the ionizer, you used a very important term; "open 
circuit".  This is the voltage when there is no current.  One often uses a 
voltmeter to measure voltage.  This instrument is designed to make this 
measurement while allowing very few electrons to flow.  This is important 
because you want to make your measurement without the meter affecting what 
you are measuring.

To illustrate this, let's go back to the battery.  Let's say you found a 
battery and wanted to know if it was still good.  You measured it with 
your voltmeter and got 1.5V.  Now, you put it in your flashlight, but get 
only a weak glow.  If you measured the voltage of the battery in the 
circuit, you would find it was significantly less.  The battery had 
the "potential" to generate 1.5V, but the chemical reaction was exhausted, 
and could not supply the current necessary to keep the bulb lit.