since graphite conducts. if spiral shape is drawn with pencil on the wall (thick , dark lines)
will a voltage difference appear between inner and outer end of the spiral when a varying magnetic field is switched on, normal to wall
my aim is to incorporate this into new mode of wireless data transmission

 

While this sounds theoretically possible, I would have a hard time imagining the pencil line as a reliable electrical conductor.

Do some experiments with a pencil line- see if you can get a reliable and stable resistance reading on a multimeter.
How would you make the final connection to this pencil line?

 

And it will likely be better the softer (more graphite, less clay) the pencil grade. Go for 9B.

 

With proper continuity, the spiral should pick electromagnetic waves and produce a signal at its ends. A wired similar but for transmitting is used in concert halls, theatres ceilings or floors to feed second program/translated audio to hearing impaired or dedicated receivers among the public.
It transmits by feeding plain audio onto the winding, no RF carrier and it is called "audio induction loop". I installed one on the floor of a 400 seat theatre loooong ago.

----> https://en.wikipedia.org/wiki/Audio_induction_loop

 

but graphite is very limited in how high a frequency you can use Because of the magnetic saturation parameter of the material.
https://en.m.wikipedia.org/wiki/Saturation_(magnetic)
can graphite be adulterated to use at high frequency?

 

How would you make the final connection to this pencil line?

at the end of line is a dark graphite circle , circular aluminium lamina stuck on it with tape act as electrodes