By Henning Umland
http://www.celnav.de/hv/hvindex.htm
“If, for example, the capacitance of C were 1000 μF, a charge of 1 mAs would result in a voltage of 1 V, independent of any current fluctuations (the capacitor voltage varies in proportion with the charge). I use two paper/oil capacitors with a total capacity of approx. 1500 μF. Thus, a capacitor voltage of 1 V is equivalent to a charge of 1.5 mAs. The time constant resulting from C and the internal resistance of the vacuum tube volt meter, VTVM (Ri = 11 MΩ), is 16500 seconds. Therefore, the capacitor voltage remains virtually constant for some time after the anode current stops flowing. Electrolytic capacitors are not suitable for such a circuit because of their relatively high and unpredictable leak current…”
From “Building a Simple X-Ray Machine”