I'm installing an Aprilaire 600a on my antique [well- 1980 or so]
Sears? furnace.

So of course it doesn't have an 'eac' or 'hum' terminal. We don't
use it in 'fan only' mode anyway, so I thought it would be an easy
task to just hook the wires to the fan relay so the humidifier is
active when the fan is running.

I haven't hooked it up yet because I don't want to fry my electronics.
So I put my tester on what I thought would be a 24v fan circuit- but I
get no reading. . .

Operator error- or wrong assumption?
Here's a picture of the Honeywell R8184 relay
http://i289.photobucket.com/albums/l.../honeywell.jpg

I expected to find 24v on 'C' and 'G' - but get nothing. I tried the
AC scale, too just to see if I misunderstood the schematic. I get a
zero reading on that scale, too.

Can anyone tell me where I want to hook the humidifier to let it know
when the fan is on?

The humidifier 'installation instructions' show two wires hooked
either to "Hum" and common- or to 'W' and common.

Thanks,
Jim