[Heathkit] O.T. Dow Key Coaxial Relay

[Kevin Ward]

You can tell an AC coil from a DC coil if you can get the coil cover off 
(without ruining it) and get a look at the coil itself.  An AC coil has 
a D-shaped ring through the core at one end.  This keeps the relay from 
chattering as the coil voltage swings through zero.  A DC coil has no 
D-ring. 

Depending on the core material, running an AC relay on DC can magnetize 
the core.  The relay may then tend to hang in the energized position, or 
may be slow to drop out. 

If the relay tends to hum, buzz or chatter when run on AC, it probably 
has a DC coil. 

Kevin  N2IE