You should note that the input resistance is constant though out the range switch. That is, the input resistance is 10Meg, whether the range switch is operated on the 1-volt position or the 100-volt position. Accordingly, the ohms-per-volt sensitivity changes with the range selected. In this example, the sensitivity is 10Meg on the 1-volt range, and 100K on the 100-volt range. This is still significantly higher than the typical 20,000 ohms-per-volt of analog VOM's of the day. What is further done usually is to use an isolation probe, where a 1Meg resistor is soldered into the probe casing. This effectively make the input resistance 11Meg on all ranges. If this high input impedance were not enough of a benefit, note that the voltage being measured is amplified by the two triode tubes. This isolates the low-resistance meter movement from the circuit being measured. The meter movement is located in the cathode of the bridge circuit, and the differential voltage is displayed. In practice, these two triodes are a single 12AU7, which is the most common tube to be seen in every VTVM. Some very old VTVM's I have use a 6SN7, but these are few and far between. The biggest benefit here is that the meter movement cannot 'load down' the circuit you are trying to measure. Therefore, voltage measurements are far more accurate than with a regular VOM. Is this increased accuracy necessary? Probably not, but it will come in handy later, as we'll soon see.
