Natural Gas furnace, Intermittant pilot detection

How is the presence of a burning pilot detected, when the only connections to the burner are:

1. A single conductor 10,000 volt wire for the spark, and
2. A single aluminum gas line tube, and
3. A chassis ground wire ?

(The controller is a Honeywell H86S 24 volt,
The gas pilot light cycles on and off, and the spark cycles on and off, both for 90 seconds, the main gas valve never turns on. After 90 seconds only the spark remains on.)

Hi, naturalgas!

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Thanks!