Why doesn't an electrometer's capacitance influence the measurement of voltage? I've read on the subject in several books, and none of them mentioned whether we can neglect electrometer's influence on the measurement of the voltage or not. Maybe my question sounds a bit stupid, but I really can't understand why they do not address the fact that electrometer has a certain capacitance, and when we connect its parts to the desired points, the electrometer and the conductor become one equipotential conductor, and certainly its capacitance no longer depends only the conductor we're testing but also on the electrometer. And we know that $q=CV$ so it influences the voltage as well. So why the measurements are still fine? Can we really neglect the capacitance of an electrometer? Or am I completely wrong here?
Thanks in advance.
 A: For low frequency measurements the capacitance in the circuit is much higher than the probe's capacitance (100pF-10uF+ vs 10pF).  The slight temporary effect of connecting the meter is quickly compensated for by the circuit because you're talking about a very small amount of charge that is pulled off in a quick transient then replaced as Art Brown pointed out.
Once you move to higher frequencies this becomes a much bigger problem.  For example at 1GHz your probe may only have 10pF of capacitance, but the circuit you are measuring probably only has 0.5pF.  The addition of probe would kill the 1GHz signal by shunting it to ground.
You could design an extremely high resistance network with very little capacitance and inject a signal that would be filtered by the addition of the probe's capacitance (forming a low frequency RC low pass filter with the probe).  However due to the normally very high impedance of the meter is is very hard to do this in any practical circuit.
However most text books avoid mentioning these effects because they are really only present under high frequency conditions.
A: You are correct:  the electrometer will "steal" some charge from the potential it's measuring.    How much effect that has on the voltage depends on the nature of the measured item:


*

*If the measured item is an isolated conductor with a fixed charge, you can calculate the voltage "error" from the capacitances as you described.  (A nice exercise!)

*However, if the voltage source can regenerate (for example, it's a rectified transformer output with a capacitive filter and a resistive load), the voltage will recover to its original value (because the electrometer draws no DC current), and there will be no voltage error due to the measurement.

