By David Lee Rickman II, W3DTV
Note: I am posting the following under Creative Commons (CC BY 4.0); just as Tesla wanted to share electricity with the world. All my possible responses to this posting are under the same license unless otherwise stated.
One of the things that powers a Teslapunk story is energy (insert groan response to pun here). Ideally, the energy source should be cheap, clean and portable. Many Teslapunkers may imagine some combination of coils and capacitors to extract energy from the aether or some motor hooked up to an alternator made from a wish alloy to produce perpetual motion. However, not all Teslapunk stories need to have a Rube Goldberg energy system. Using the KISS (Keep it Sweet and Simple) principle one can imagine a small discrete component which can be inserted to a circuit and act like a battery. What is it? The negistor (negative resistor).
A negative resistor is the opposite of the regular (positive Ohmic) resistor you will find at the local Rat Shack. Unlike conventional resistors which drop voltage by dissipating heat a negistor would converge ambient energy from its surroundings to produce a voltage increase above what the source supplies.
According to conventional physics a negative resistor circuit component is impossible because it theoretically violates the second law of thermodynamics. However, Nikola Tesla took an opposite view. In his essay "The Problem of Increasing Human Energy" Tesla argued that energy could indeed be extracted from an ambient medium.
For a possible Teslapunk story the negistor is one option for a cheap, clean and portable energy source with a legitimate connection to Tesla's research. By connecting a dry cell battery to enough negistors your character can get large amounts of power. From there, your Teslapunk story can explore the other possibilities. For example, is the product of a negative resistor and a positive capacitor an absolute value? Maybe the time constant of an RC circuit can be negative. That might mean getting energy from the future because the capacitor is charging in negative time. From that idea a backward-time switch can be constructed or a filter can be made to receive negative frequencies.
If as an author you think the product of negative Ohms and positive Farads is an absolute value then you still have a way of self-charging your Teslapunk ray gun.
Never underestimate the power of pessimistic resistance (insert last groan response here).