Here is an example that illustrates how little current a grid conducts when it is substantially more negative than the cathode. If a 9-volt battery is connected to a triode so that its negative terminal attaches to the grid and its positive terminal attaches to its cathode, while the cathode is grounded and its plate is connected to a high voltage power supply, the battery's life expectancy would probably prove no shorter than that of its brother still shrink-wrapped at the store. Yet, the battery is an essential part of the circuit.

    In other words, the grid is like the valve in a water faucet; so much like it, in fact, that the vacuum tube is often referred to as a valve. Because turning a water faucet on and off requires so little effort, we cannot see the splendor of the stunt. So, instead, imagine a large dam and its water valve; it is a huge reservoir of potential energy, yet one individual can turn the handle (the valve) that releases a dangerous flood. The amount of work required to turn the handle is nothing compared to the water's release and once the effort has been expended opening the valve, it remains open without any further expenditure of work on our part.

     Like the amount of work required to release the dam's flow, most of the work required to adjust the triode's current came at the beginning. Charging a capacitor requires energy, just as filling a swimming pool requires water and pressure. In this example, this one-time connection results in an insignificant amount of work. Make and break this same connection a million times per second and the battery will be taxed and depleted, as now the recharging of the capacitance represents much more work. (Imagine what your hands would look like after turning the water on and off a million times per second.) Not all tubes would require the same amount of work, however, as each type has a differing amount of capacitance.
    In the battery-biased circuit shown above, the circuit without a plate resistor, the varying amounts of capacitance would be the only issue, but in an amplifier circuit, wherein the plate works into a load (such as a resistor or a transformer), the amplification realized by the triode becomes crucial to determining the amount of work required to recharge the capacitance.

    In much the same way as the dam's flood valve, the triode's grid presents an extremely light load, considering the large resulting effect. Unlike a transistor's low-impedance base, the grid's input offers so great an impedance that one of the triode's last jobs, before being replaced by solid-state devices, was as the input device on voltage meters and oscilloscopes, where its high-input impedance prevented excessive interaction with what was being measured and its ability to survive high voltage mishaps added reliably to the test instruments.

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