The following explanation of the basic principle of harnessing free or radiant energy from the ether was published in 2001 by Peter Lindemann in his book:

The Free Energy Secrets of Cold Electricity
By Peter Lindemann, D.Sc.
© 2001, 2004

The Electro-Radiant Event is produced when a high-voltage, direct current is discharged across a spark-gap and interrupted abruptly before any reversals of
current can occur.
This effect is greatly increased when the source of direct current is a chargedcapacitor.
The Electro-Radiant Event leaves wires and other circuit components perpendicular to the flow of current.
The Electro-Radiant Event produces a spatially distributed voltage that can be
thousands of times higher than the initial spark discharge voltage.
It propagates instantaneously as a longitudinal, electrostatic “light-like ray”
that behaves similarly to an incompressible gas under pressure.
Electro-Radiant effects are solely characterized by impulse duration and
voltage drop in the spark-gap.
Electro-Radiant effects penetrate all materials and create “electronic
responses” in metals like copper and silver. In this case, “electronic responses”
means that an electrical charge will build up on copper surfaces exposed to
Electro-Radiant emissions.
Electro-Radiant impulses shorter than 100 microseconds are completely safe to
handle and will not shock or cause harm.
Electro-Radiant impulses shorter than 100 nanoseconds are cold and easily
cause lighting effects in vacuum globes.

Comment:
Challenges in building such a device lie in the fact that high voltages can be dangerous or even deadly, and not many possess the skills to handle them safely. Additionally, the circuits need to exhibit specific resonance properties, and achieving resonance in circuits is a complex topic.

Nevertheless, there have been researchers who have attempted to spread awareness by sharing schematics. However, the condition under which they were allowed to share this information often required them to omit certain critical details, making it impossible for the average person to replicate the device without those missing components. Despite this, these researchers hoped that by sharing incomplete knowledge, others might be able to figure out the missing parts.

There are many highly complex schematics available, but they are typically impossible for the average person to replicate. One exception is the work of Don Smith, who shared simple and easy-to-build devices with high power output. His work can be explored further here:
Don Smith - archive.org
Don Smith - free-energy-info
You can also find the manual on the internet by using the key words "Don Smith" "free energy" "pdf"

After researching the Don Smith device for many years, we have found the missing clues in forums, where some successful researchers shared the critical missing information. The Don Smith device will be a central study object of this forum, and we are very confident in our ability to replicate the device and share it with others here.