non-amplified analog multimeter. To avoid the loading of the measured circuit by the current drawn by the meter movement, some analog multimeters use an amplifier inserted between the measured circuit and the meter movement. While this increases the expense and complexity of the meter, by use of vacuum tubes or field effect transistors the input resistance can be made very high and independent of the current required to operate the meter movement coil. Such amplified multimeters are called VTVMs (vacuum tube voltmeters), TVMs (transistor volt meters), FET-VOMs, and similar names. Because of the absence of amplification, ordinary analog multimeter are typically less susceptible to radio frequency interference, and so continue to have a prominent place in some fields even in a world of more accurate and flexible electronic multimeters. Probes A multimeter can use many different test probes to connect to the circuit or device under test. Crocodile clips, retractable hook clips, and pointed probes are the three most common types. Tweezer probes are used for closely spaced test points, as for instance surface-mount devices. The connectors are attached to flexible, well insulated leads terminated with connectors appropriate for the meter. Probes are connected to portable meters typically by shrouded or recessed banana jacks, while benchtop meters may use banana jacks or BNC connectors. 2 mm plugs and binding posts have also been used at times, but are less commonly used today. Indeed, safety ratings now require shrouded banana jacks. The banana jacks are typically placed with a standardized center-to-center distance of , to allow standard adapters or devices such as voltage multiplier or thermocouple probes to be plugged in. Clamp meters clamp around a conductor carrying a current to measure without the need to connect the meter in series with the circuit, or make metallic contact at all. Those for AC measurement use the transformer principle; clamp-on meters to measure small current or direct current require more exotic sensors, such as; hall effect based systems that measure the nonchanging magnetic field to determine the current. Safety features Most multimeters include a fuse, or two fuses, which will sometimes prevent damage to the multimeter from a current overload on the highest current range. (For added safety, test leads with fuses built in are available.) A common error when operating a multimeter is to set the meter to measure resistance or current, and then connect it directly to a low-impedance voltage