A key component in most lamps is threaded steel tube, often brass-plated, called “lamp pipe.” The standard size is 1/8 IP; the IP stands for “iron pipe.” The sizes belong to the 19th century Briggs Standard of Wrought-Iron Pipe Dimensions, to which seamless brass and copper tubing was also made.¹ Although 1/8 IP tube is by far the most common size in lamps, three IP and one non-IP size are used:
|1/8 IP||0.405 inches outer diameter, fits in a 0.416 (or a 7⁄16) inch hole. External thread 27 threads to the inch.|
|¼ IP||0.540 inches outer diameter.|
|3/8 IP||0.675 inches outer diameter.
This was the size of the outlets for gas lighting; the system of sizes used for the pipes that brought gas to gas lights was adopted for electric lights. In older ceiling fixtures (the outlet box is perfectly round instead of square) the light is attached to a 3/8 IP stud, just as a gas light was. Today an adapter, called a “hickey,” is used to get from the 3/8 IP stud to the fixture's 1/8 IP.
In addition to the IP sizes, a smaller thread is available,
|¼-27||Unlike the IP sizes its outer diameter actually is ¼ inch, but its screw thread is not standard. The United National Fine thread standard for quarter-inch bolts and nuts is 28 threads per inch. The standard thread for 1/8 IP pipe, however, is 27 threads per inch.|
The locknuts, reducers, adapters, and so on for use with lamp pipe are specialty items; ordinary nuts won't work. Fortunately many hardware and craft stores carry a large selection of lamp parts.
Harps, which connect a lamp shade to the tube screwed into the bottom of the lamp socket, are made in heights from 6 to 12 inches in steps of ½ inch, and then by whole inches to 15 inches. The stud at the top of the harp, onto which the finial is screwed to hold down a shade, has a ¼-27 thread.
Machine screws in lamps are almost always 8-32.
1. Report of the Committee on Standard Pipe and Pipe Threads.
Transactions of the American Society of Mechanical Engineers, volume 8, 1887.
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Last revised: 11 October 2006.