Negative resistor is real thing, easy to build from OPAMP and 3 resistors or even from two transistors and few resistors. There are also tunnel diodes that have negative resistance segment on their Volt-Amp characteristic (nearly all 10GHz and higher generators build using tunnel diode working in negative resistance mode). Just search it - you will get a lot of circuits and explanations of how it works. Problem is that negative resistor always consume more power to work than you will be able to get from that negative resistance.
Saying that it could be so doesn't show how to do it. I doubt that Kron actually had any secret "negative resistor" component.
Negative resistor is real thing, easy to build from OPAMP and 3 resistors or even from two transistors and few resistors. There are also tunnel diodes that have negative resistance segment on their Volt-Amp characteristic (nearly all 10GHz and higher generators build using tunnel diode working in negative resistance mode). Just search it - you will get a lot of circuits and explanations of how it works. Problem is that negative resistor always consume more power to work than you will be able to get from that negative resistance.
That sounds like a transformer, If you hide the other loop of wire, a transformer looks like a power source.