FET and vacuum-tube voltmeters

It was mentioned that a good voltmeter will disturb the circuit under test as little as possible, and this requires that the meter have a high internal resistance. Besides the electrostatic type voltmeter, there is another way to get an extremely high internal resistance. This is to sample a tiny, tiny current, far too small for any meter to directly indicate, and then amplify this current so that a meter will show it. When a miniscule amount of current is drawn from a circuit, the equivalent resistance is always extremely high.
The most effective way to accomplish the amplification, while making sure that the current drawn really is tiny, is to use either a vacuum tube or a field-effect transistor (FET). You needn’t worry about how such amplifiers work right now; that subject will come much later in this book. A voltmeter that uses a vacuum tube amplifier to minimize the current drain is known as a vacuum-tube voltmeter (VTVM). If an FET is used, the meter is called a FET voltmeter (FETVM). Either of these devices provide an extremely high input resistance along with good sensitivity and amplification. And they allow measurement of lower voltages, in general, than electrostatic voltmeters.