I know I'm running the risk of being sanctioned for this question as it isn't strictly a tractor ? but I think it has the same principles. It's actually a 1989 Wrangler. (Did I cut my own throat by telling you that?)
Thanks

This secondary induced voltage is not just a "single pulse" either. The collapsing field is energizing the coil too, though much less than when the coil was fed from the battery, and as this second field reaches its peak saturation moment, it too collapses, falling back through the coil and generating a "tertiary induced voltage", which by the way, matches the voltage of the electrical system now, and so on until all EMF activity ends.
This will show up on a very sensitive, analog type, DC volts scale as a wildly fluctuating needle (for a fleeting moment).
On an economy digital meter set to DC volts, you might see the polarity icon change from + to - and back a couple of times, along with the value trying to change, or just going blank as it can't react fast enough to show a value.

But it would seem that simply turning on the key switch would not provoke the operation of any solenoids, relays, or motors unless the operator moved a switch or control setting, or if there was an automatic process ongoing when the key is turned to ON.

Does this Jeep perhaps have a heater/AC fan that runs whenever the key is on (even if the manual fan switch is OFF). I know many GM products had cabin fans that ran on low speed whenever the key switch was on, even if the fan switch on the control console was in the OFF position.

Or, it could be the battery charger voltage limiter chopping the current when the battery/system voltage rises to to the voltage limit setting of the charger.
If this is occurring, it could theoretically reach a point where this "cycling" of the voltage limiter would occur every "7 seconds or so" (or other interval) as the battery and the charger try to reach their own "Nirvana".

If it is in fact due to the battery charger voltage limiter action, AND the fluctuations occur only when the key is ON, the vehicle alternator may be the source of the spike.
With the key ON, the vehicle voltage regulator will energize the alternator FIELD windings whenever the vehicle electrical system voltage is less than the voltage limit setting of the regulator. Any change in the voltage level of the vehicle electrical system (either higher or lower change) will cause the alternator output windings (stator coils) emit a "pulse" of current, even though the alternator rotor is not spinning.
To visualize that, think of the non-moving alternator as if it was an ignition coil with the rotor (field) windings acting as the PRIMARY coil windings and the stator windings acting as the secondary windings. Any change of voltage level in the rotor windings will produce a pulse of induced current in the stator windings.
On an analog voltmeter the pulses would appear as a fluctuating spike similar to AC.

It sounds like the old fashioned automotive clocks that would "wind up" every few minutes.

There are hour meters still made and used today that have mechanical movements driven by main spring and motor for the same reason, system voltage variations.

But would a 1989 Jeep Wrangler feature an "old school" analog clock?

To isolate the alternator from this scenario, disconnect the wiring from the alternator after first removing the battery negative cable.
Tape the end of the battery wire of the alternator.
Reconnect the battery negative cable and hook up your charger as before.
Wait and watch for the fluctuation to occur.

A "leaky" capacitor could be drawing much more frequently.???
I know that's kind of out there, but.....

You can probe the wires that connect to the wiper motor to see what sort of readings you get at different settings of wiper speed and parked.
As I said above, you should have one wire that remains "hot" even when the wiper switch is off. Intermittent wipers will send the brief power pulse to the motor on the "slow speed continuous" wire. Once you identify which wire works low speed continuous, then check that wire in the intermittent positions to see if this is the "spike" you have been seeing.

By the way, I have been wowed by the breadth and depth of your knowledge and willingness to help anyone over the couple of years I have been a forum member. This isn't meant to be patronizing flattery. You are amazing and a godsend. Thank you from all of us that you have been so patient with as well. I hope to be able thank Bill as well. He is also amazing. (And I'd really love to know how you've acquired so much knowledge about all things mechanical!)

Even when you experience things that don't seem to matter that much to you, you can still observe.