Form 109 Top Mount Streetlight
Many streetlights designed for bracket arm mounting were also available in a version for vertical pipe mounting. The same optical assembly (reflector and refractor) would be used with a choice of head, or lamp socket enclosure, with the appropriate mounting above.
This General Electric streetlight was salvaged from the former Boston Central Artery. It was installed in the mid 1950's when the elevated roadway was constructed. It was mounted in the girderwork under the elevated roadway to illuminate the surface street passing underneath. In this installation the conduits for the street lighting power lines were mounted on the underside of the roadway deck with junction boxes at intervals with a vertical pipe dropping down to the height desired for the light fixture.
At first we thought that the head was a custom designed piece for the vertical pipe mounting and the Form 109 globe. Upon closer examination we find that the "hood" above the reflector globe consists of two aluminum pieces. The lower piece is an adapter ring that has a NEMA (National Electrical Manufacturers' Assn.) standardized top opening and a Form 109 lower opening.
The top cap was made by GE and has Design Patent No. 100102 assigned to GE. It can be thought of as a NEMA head although it is much less tall and has very little interior volume.It is 3-7/8" high including the female threaded top collar, and has mounting tabs for a lamp socket assembly. There is about 2-1/4" of height inside below the socket mounting pads, .Because the mercury lamp for a Form 109 is mounted inside the globe, there is no significant space requirement above. The threading is for a 1-5/8" OD pipe.
The adapter ring appears to be the same adapter used to mount Form 109 globes to incandescent lamp NEMA heads. It adds 3-1/2 inches to the overall height of the fixture. There were no markings on the ring, although we would expect it to be a GE product since both the cap above it and the globe are GE products. Outside diameter of ring, 10-3/8 in.
Two aluminum brackets clip over a NEMA head refractor mounting ears and are held onto the adapter ring with a pair of bolts each.
Because the adapter ring is "permanently" fastened to the top cap, relamping is accomplished by unsnapping the globe, as is done for side mount Form 109 fixtures.
This unit came with a 250 watt mercury lamp. The socket bracket is mounted to the inside of the adapter ring using the same bolts that hold the refractor hinge clip. The socket assembly is too short to accommodate a 400 watt lamp. We currently believe that the socket for a 400 watt lamp must be mounted inside the globe, and some Form 109 globes (not this one) have a stud for this purpose under where the outside hinge is mounted.
Complete parts list for vertical pipe mount Form 109
Small cap head with NEMA standard lower opening and threaded top opening for mounting pipe
Adapter ring with NEMA top opening and Form 109 bottom opening
Terminal Strip, mounts inside top edge of adapter ring.
Circular reflector, mounts in adapter ring lower opening
Arc shaping coil (optional, none in this unit), mounts above circular reflector.
Form 109 horizontal lamp socket assembly.
Form 109 globe reflector/refractor
Overall height, 21". Estimated weight, 19-1/2 lb. without lamp or arc shaping coil. We were unable to weigh the top cap as we could not detach it from the mounting pipe fragment, weight estimated as 2 lb. Weight of adapter ring with mercury lamp accessories attached, 4 lb. Weight of reflector/refractor globe, 13-1/2 lb.
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