I have not tried this yet, but I will. My first guess is that the blackened
membrane itself heats/cools and therefore moves from expansion contraction
(rather than the enclosed air heating/cooling). The pickle jar cannot
appreciably do this because the glass is too thick and rigid. I continue
having difficulty believing the air near a blackened surface can heat/cool
fast enough to respond sufficiently to a small 120-Hz fluctuation in radiant
energy.
I will also try the following if you have not... does a clear or white
stethoscope membrane produce the hum?
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817
Bringing the black membrane of a stethoscope very close to a 7.5 W
incandescent bulb produced a quite distinct hum in the stethoscope, which
surprised me. Moving the stethoscope away, or shielding it with a piece of
paper proved that the hum was due to the flickering of the bulb.
I'll have to buy a jar of pickles.
I wonder if the stethoscope is a better "matching transformer" than the tube
you were using???
Br. Robert W. Harris
Catholic Memorial High School
rwharris@cath-mem.org http://www.cmphysics.org