I assume that you are using this on 120 volts AC (line voltage in the
US).  Place a _good_ Mylar capacitor, 0.1 mfd @ 600 volts ( at least
600 volts -- even better is 1600 volts!) across the terminals of the
switch.  Use Sprague "orange drop" or USA-made equivalent -- don't use
the imported junk)  This should knock out the arcing.

If you are on 220 volts use the 1600 WV DC capacitor.   A TV shop should
have these, but again be wary of junky import stuff.  I have had
capacitors short out with a big bang.  Also observe proper AC safety
precautions in mounting this capacitor so the leads won't touch
anything or anyone.

Scott Olson - Missoula Montana ( much needed rain finally, 2nd day)

 [ There should also be a small resistor in series with the capacitor
 [ (making the circuit a 'snubber network') to protect the switch from
 [ the capacitor discharging current when the switch closes.  5 ohms at
 [ 2 watts is probably okay for a switch rated at 250 VAC operating on
 [ 120 VAC mains.  -- Robbie