Mine is a battery and light bulb apparatus radiating energy [mostly] isotropically.
I originally used it to demonstrate a thought experiment (borrowed from another person) which shows that as an object radiates energy it loses a corresponding amount of mass -- even an apparatus as simple as this one.
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I had once considered a different avatar of a sacrificial Van de Graaff Generator Monkey.
[Figure 1: Sacrificial Van de Graaff Generator Monkey]
The idea is that if there was a velociraptor attack it could be used to trigger an alarm system thus giving me approximately 0.7 seconds of more life, before being devoured by the velociraptor.
It operates by me first carrying around the Van de Graaff Generator Monkey day and night (even on the bus), keeping it powered on and activated all the time. Even to bed. You can never be too prepared. I got used to the constant noise and electrical shocks after awhile. I even sing a little ditty along with the rhythmic clanging sounds. It's sometimes popular on the bus.
Once a velociraptor breach is detected, I'll toss the Van de Graaff Generator Monkey at the velociraptor where it will be gobbled up immediately, giving the velociraptor a net electrical charge.
[Figure 2: Security breach]
Once the velociraptor has a net electric charge, it will create a magnetic field as it moves toward me at 20 m/s (max). This magnetic field is used to trigger alarms inside and outside of my house to warn my neighbors within a 0.9 mile radius, before I get gobbled up by the velociraptor.
[Figure 3: Alarm system]
In the end though, I just went with the light bulb out of laziness.