[3dem] Slowly varying magnetic fields

[John Rubinstein, U of T]

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