Exactly.
Nobody would connect 27v to the base of the transistor. But there is a capacitor of unknown value to allow spikes to pass to the base of Q1. Remembering that Q1 is the first stage of amplification of the big loop, the question then becomes... what kind of spike could come from the rod at the end of the ion chamber to combine with the loop signal?
When we look at the ion chamber, it appears to have a ground and a gold leaf that is connected to a 5v square wave at 10 Hz. The brass rod connected to Q1 is separated from the gold leaf conductor by about 1/2 cm.
Does the 5v square wave pass to Q1 via C20? Is there some inductive or capacitive coupling between the gold leaf and the brass rod? Maybe EMI from the edges of the square wave?
What about the noise from the square wave switching in the Atmel processor located inside the big loop?
I remember picking up calculator clocking from a smaller loop at 5 cm distance from the calculator.
In fact I didn't need a loop. Any conductor the size of your thumb will work to pick up the pulses so I could display them on an oscilloscope.
I would imagine there is some artifact from the 10 Hz signal entering into the first stage simply because of the size of the big loop, and because this is the first stage of amplification before filters could lose the 10 Hz noise.
The biggest problem that stops us from picking up the 10 Hz is that it is such a tiny signal with no measurable power being consumed, and there are likely many sources of noise in the air much stronger than the 10 Hz noise.
The base of the transistor is also low impedance. But I am guessing you would see the 10 Hz clocking mixed with whatever other signals you find if you connected your oscilloscope to the base of Q1.
This is not to say the 10 Hz is a gold signal or gold frequency, only to say that it is not impossible that this 10 Hz could be transferred to the signal at Q1.
It still seems strange to send the two signals to a single transistor. When we look at it, we see these are two different kinds of noise signals being combined... broadband VLF and circuit noise... combined to be amplified.
Best wishes,
J_P