I think you nailed it, Brian. Thanks for the research.

Dave N9ZC

On Sun, Apr 9, 2023, 10:08 PM Brian Harrison <briankn4r@gmail.com> wrote:
Hi Dave,



They are called resistance lamps (sometimes called ballast lamps) and they are inserted in series with ringing supply leads (manual or machine) so that when a called phone goes off-hook during the ringing cadence, a short circuit is not presented across the ringing supply. 

The ones in your photos were probably from a small manual or automatic PBX installation, and they are mounted in type 105 boxes. The ones on my 701 PBX are mounted on a 23" plate above the ringing machines. 

The earlier ones had screw bases, and the later ones had solder lugs in Bakelite (or some other kind of black plastic) bases.

Type 12 

Type 13 

best,
brian
kn4r

On Apr 9, 2023, at 6:51 PM, Dave Merrill <r390a.urr@gmail.com> wrote:

Anyone know what this is for? Some kind of ballast tube, made in 1963.  Searching the FSN came up empty

Dave N9ZC