voltage of capacitor

An ordinary voltmeter has a finite resistance. If you connect such voltmeter across the points A and B, the circuit becomes a two resitances series circuit, with one resistance being R and the other resistance is that of the voltmeter. A current would flow through the circuit.
Because the capacitance can be treated as an 'open-point', the current passing through, and hence voltage across, the voltmeter resistance is then determined by the resistance R and the voltmeter's own resistance. Thus the original situation, where the capacitor is fully charged and there is no current flowing, is altered. The voltage measured by the voltmeter would be different and is not measuring the voltage across the capacitor in the original situation.
In order to measure the voltage across the capacitor accurately. The original situation (i.e. no current flowing in the circuit) must be maintained. The could be done by connecting an 'ideal voltmeter' across the capacitor. An 'ideal voltmeter' has infinite resistance and draws no current. Thus, its connection to the circuit would not change the original situation.
An electrometer is a type of 'ideal voltmeter'. Similar to a CRO, which also acts like an 'ideal voltmeter'.

你需要用一種叫真空管電壓表或電子管電壓表Vacuum Tube Voltmeter (VTVM)來量，因爲此種電壓表内阻非常高，或者用示波器Oscilloscope來量度可以。

圖片參考：http://i1.ebayimg.com/01/i/000/f8/29/bfc2_1_sbl.JPG

可以，點解唔可以，不過AB電壓會因R作用緩慢上升，最終會等如dc。但當dc斷開時，電容會向電壓表放電，如果嗰時去量度就會失準，所以要採用較高內阻電壓表量度。