[3dem] Slowly varying magnetic fields
John Rubinstein, U of T
john.rubinstein at utoronto.ca
Wed Jun 3 12:23:23 PDT 2009
Dear Colleagues,
We recently identified a slow varying magnetic field (e.g. changes on
the time scale of ~1 second) in our microscope room. Looking at our FEI
engineer's EMF meter, the minimal 60 Hz AC fields in the room appear to
be carried on a (much larger) background that resembles the rising and
falling of an ocean surface (but less regular). This pattern would be
quite pretty to look at if the fields did not cause random displacement
of our electron beam of up to ~5 mm on the phosphor screen at 600 kx
magnification (Tecnai F20 operating at 200 kV). These fields were not
present 2 years ago when we installed the microscope. We are not near
any elevators and our room is on the 3rd floor so we know we are far
from any subway tracks. These "DC" fields are strongest as you go up in
our room, but are stronger in our room than in the room above ours. The
only ductwork and wires in our ceiling are those we put in for our air
conditioner and dehumidifier and the fields persist with this equipment
turned off. Naturally, I am concerned that something has changed far
away in the building causing these effects and that the fields are being
transmitted to our room via metal beams or rebar.
Has anybody come across this sort of problem before, identified the
source of the fields, and/or solved the problem in some way? Any
insight from experience would be greatly appreciated.
Best regards,
John
--
John Rubinstein
Molecular Structure and Function Program
The Hospital for Sick Children
555 University Avenue, Rm. 3330
Toronto, ON
Canada
M5G 1X8
Tel: (+001) 416-813-7255
Fax: (+001) 416-813-5022
www.sickkids.ca/research/rubinstein
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