Van der Graaf´s equivalent circuits for series and shunt modulators

It is shown that the equivalent circuits used by van der Graaf, while giving perfectly correct results for the problems to which he applied them, lack general validity, even within the conditions assumed by him. In particular they appear to demonstrate that it is possible to obtain a constant input resistance (i.e. constant with respect to time) in a series or shunt modulator, whereas clearly such a result is impossible in reality. An alternative system of equivalent circuits is shown to avoid the difficulty. An explanation is given.