A vacuum tube voltmeter. The VTVM uses a vacuum tube to amplify incoming voltages, greatly reducing the amount of load placed upon the circuit under test. A VTVM has a very high impedance.
Some VTVMs also measure high resistances, using the tube to amplify the current passed through the circuit under test.
The modern equivalent of the VTVM is a FET-amplified solid state meter. All digital multimeters and some newer analog meters use a FET.