An old ratty drill, with universal (brushed) motor works well.

I built an impulse tester years ago that I use in a much more controlled
manner, but it's a pretty lethal device.  For small discharges, the main
unit charges through a 40 meg high voltage resistor into a 100pF capacitor
made from circuit board material, and that cap is discharged through a 1500
ohm resistor into the circuit.

For the harsh stuff, I use the main unit directly, with about a 1" air gap,
discharging it's main caps.
The main unit is an encapsulated DC-DC converter with 0-30,000V output,
charging into three 0.01uF 30kV low inductance caps.  The discharges are
loud enough to require hearing protection.  The charge stored in the caps
and residual inductance gives me a few cycles at about 2 MHz, with a
radiated power of about a million watts, give or take.  Small wire loops can
be used to get a larger magnetic field.  It puts audible pulses into radio
and wired phone systems for quite a distance, and the charged caps are quite
lethal.  I only use it with an assistant present, who knows CPR, and I
strictly obey the "one hand" rule.

Basically, start the target system, and the sparker, and bring them closer
and closer till the target has an upset.  Apply whatever you think will
help, and try again, noting the difference in distance before the upset.
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