Standardized Streetlight Socket Assembly
In street lighting terminology, the "head" is the part of the lighting fixture that is attached to the bracket arm and that usually encloses the lamp socket.
Attempts to standardize the head go back to at least 1935, possibly earlier. By having a standard head, a choice of reflectors and globes can be interchanged without re-installing the entire fixture. Back then, almost all bracket arm and pendant (hanging) streetlights used incandescent electric lamps with the base mounted upwards. A relatively simple cylindrical head therefore served almost all needs.
The head illustrated here was made by General Electric and has Design Patent No. 100102 assigned to GE. (The design patent does not include the details and claims the latter of which undoubtedly describe standardizing dimensions and easy to assemble latch assemblies.) 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, therefore most of the socket assembly must occupy the reflector area underneath. The bottom mounting has a 6-1/2" outside diameter and a 5-3/4" inside diameter. Although the bottom opening has the standard dimensions for the most common reflectors and globes, a standard socket assembly does not match the mounting hole spacing. The top threading was for a 1-5/8" OD (outside diameter) pipe.
This head was taken from a General Electric mercury fixture, where the mercury lamp was positioned inside the reflector globe and therefore space inside the head was not a requirement. It was made around 1955 and the fixture was installed on or about that date.
The patent drawing shows the two outside tabs diametrically opposite each other to which the optical assembly is clipped on or snapped on. One side shows a latch with compression spring that is in use to this day. The other (left) side shows a simpler ring fastener which could be used. In practice, latches as illustrated on the right side are used on both sides to achieve a tighter more watertight and more insect proof joint.
We have also shown an example of this head with a refractor globe attached.
With the trend towards larger higher wattage lamps for street lighting, the head quickly evolved into a taller cylinder, approximately 8 inches high, with more room for the lamp socket assembly (bottom center inset). This permitted more shallow reflector assemblies with improved optical characteristics.
The taller 8 inch, head based on the tab reflector mounting shown here is referred to as a NEMA head, after the National Electrical Manufacturers' Association which adopted it as one of their standards.
NEMA heads are still being made, and nowadays are most commonly used for yard lights.
Text of the design patent document (copied off the Internet):
UNITED STATES PATENT OFFICE
100,102 Design for a Lighting Unit
Cromwell A. B., Halvorson, Lynn, Mass., assignor to General Electric Company, a corporation of New York
Application December 19, 1935, Serial No. 60,177
Term of patent 14 years
To all whom it may concern:
Be it known that I, Cromwell A. B. Halvorson, a citizen of the United States, residing at Lynn, in the county of Essex, State of Massachusetts, have invented a new, original, and ornamental design for a Lighting Unit, of which the following is a specification, reference being made to the accompanying drawing, forming a part thereof.
The single figure in the accompanying drawing is a side elevation view of the lighting unit embodying my new design.
The ornamental design for a lighting unit as shown.
In witness whereof I have hereunto set my hand this ninth day of December, 1935,
CROMWELL A. B. HALVORSON
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