The inductor of the Joule Thief can be made in many different ways. Instead of the cored inductor, Sohei Thoth has been exploring "wireless" versions using air core coils, bifilar wound.
"Wireless" isn't really correct terminology; I think "coreless" describes it better. Certainly there is a lot of wire in these JTs!
I made some bifilar pancake coils for another project, and I decided to see if they would also work like Sohei's coils. They do, excellently, and the addition of the 70 nF capacitor across the 1K base resistor at the 2n2222a transistor, the range is extended and the lights are quite brilliant.
Not especially efficient; I estimate the current to average about 20-25 mA in the most brilliant mode, using the 1.2 volt NiMH rechargeable battery as the power source.
I illustrate the distance, and the effect of turning one coil around, and the effect of switching in and out of the 70 nF capacitor.
Thanks, Sohei, for the interesting ideas!