Radio-Frequency Pickup
If the spectrophotometer is being operated close to a transmitter, spurious positive or
negative deflections of the microammeter needle may occur. It is possible to eliminate, or
decrease the magnitude of, these deflections by keeping the two leads connecting the microammeter to the spectrophotometer as short as possible, and by intertwining the leads.
Grounding the spectrophotometer case will also help eliminate radio-frequency pickup. On some
instruments the cover does not make electrical contact with the instrument base. This lack of
contact can cause problems with power-line as well as radio-frequency pickup. The electrical
contact between base and cover can be improved by placing a “star” washer under one of the nuts
that screw on to the threaded posts that pass from the base through the cover. Verify that the
connections – “hot,” neutral and ground – to the AC (Mains) power supply are correct. It is
possible for the instrument to operate with a broken connection to neutral or ground, but be very
sensitive to electrical interference.