if the voltage drop across the ammeter is neglected, which is usually allowable
due to the small terminal impedance of such instruments. For d.c. voltage
measurements, average current-indicating instruments such as moving coil or
equivalent electronic meters are used giving the arithmetic mean value of V
according to eqn (3. 10). Less recommendable is the measurement of r.m.s.
values as the polarity of the high voltage would not be shown. Fundamentally the time-dependency vt according to eqn (3.9) could also be measured by, for
instance, an oscilloscope. The difficulties, however, in treating the resistance
R as a pure resistance are limiting this application. This problem will be
discussed later on. It is recommended that the instrument be protected against
overvoltage with a glow tube or zener diode for safety reasons