Autos that had the igniton points and condenser could be checked this way. Voltage source on left is connection to primary lead going from coil to distributor with an alligator clip and second alligator clip to go to car ground connection.
This idea might adapt to other equipment. This mechanic's idea to set in car driver's seat and let the clicks tell the engine missing tale. Listen to clicks in radio earphone as engine speeds up and clicks merge and into a buzz. If sound skips it indicates ignition system is at fault. If missing engine is not heard the condensor(capacitor) in ignition is bad and the auto ignition condenser in the tester is doing the job.
I added resistor to circuit to match speaker levels.
