One solution to the above problem is to make the needle use as little energy as possible to move. However, there is a limit to how sensitive this kind of voltmeter can be. When vacuum tubes and transistors were invented, it became possible to build electronic amplifiers. By using an amplifier, a voltmeter can measure "very" small voltages from "very" weak sources. Modern voltmeters and multimeters normally have this kind of amplifier in them.