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J-38 Keys
Telegraph Keys used during WW II
by the US Army Signal Corps

If you already know the basics of J-38 keys and want to skip the intro to get directly into the heavy "rivet counting" click here for more than you ever wanted to know about J-38 keys.

During the years between WW I and the Korean War the US Army Signal Corps designated many of their telegraph (or Morse code) keys with a "J" prefix followed by a number. The known numbers range from J-1 to J-51. All of these keys are considered collectible, and many of the WW II models are still in everyday use by Amateur Radio (Ham) Operators. One of the most popular of the "J" keys is the J-38.

Typical J-38 Key

The J-38 was designed to be used with a Signal Corps radio/telegraph operator training set. The strange looking binding posts and shorting strap at the back of the key were used to connect a set of headphones to one side of the key ("tel") and the other side of the key ("line") to the training set. The effect was to put the key and headphones in series. Click here to see a description of the set up and key circuit (from War Department Technical Manual TM 11-432, February 2, 1942).

The circuit closing switch on the right side of the key was used to set the key for listening (switch closed) or sending (switch open). The other odd piece of hardware, a little eye screw on the left side of the key, was used to tie a stay cord or lanyard from the headset leads to the key. Click on the pictures below to see an example.

There are often orange colored numbers and letters stamped on the bottom of J-38's, sometimes inside circles or squares. These stamps were used to indicate that the key had been approved by a Signal Corps inspector. The letters MFP are also sometimes found on J-38's indicating that the key had been treated for mildew and fungus prevention.

J-38 with phones

Closer view of pins

J-38 keys were manufactured by many companies during and after WW II. Generally the maker's name is not on the key. The notable exception is The Lionel Corporation, whose name and/or logo is cast into both the frame and the base of their J-38's. Many of the original boxes have survived, however, so it is possible to identify some of the manufacturers. Click on the small images or the blue links below to enlarge the photographs.

American Radio Hardware Co. Inc. J-38

The McElroy Mfg. Corp. J-38

The Lionel Corporation J-38

Radio Essentials Inc. J-38

Cook Electric Co. J-38

Signal Electric Mfg. Co. J-38

J-38 keys are sturdy, handsome, and have a good "feel" when used for sending code. With a little patience an excellent key can be purchased for a reasonable price at flea markets, hamfests, and on-line auctions. If you don't mind disassembling and cleaning the key yourself you can do even better.

Unless you buy the key from an experienced key collector, do not take the seller's word that the key "seems to be complete", or "appears to have no missing parts". Click on the small photo below to get an idea of the number of parts that there are in a J-38.

J-38 Key Detail (Lionel)

There are many differences between J-38 keys made by different companies, and even by the same companies at different times. These variations, however, are restricted to the use of different types of metal and slightly different shapes and stamping of the binding posts, frame, and base. All WWII J-38's originally had all the components shown in the pictures above.

J-38 keys were used by all branches of the military, and are sometimes found with a "Navy knob" as shown in the picture below. I have talked to retirees from the U.S. Navy who have confirmed that they used J-38's with this type of knob.

J-38 With Navy Knob

Several people have sent me email asking how I clean my J-38 keys. I have written a short description of the method I used for the keys shown on this web page. To see the description, click here.

And finally, what about the genealogy of the J-38. Take a look at the pictures below of the World War One (WWI) J-2 key. It is my speculation that the J-2 is the father (or perhaps the grandfather) of the J-38.

Click to enlarge

Click to enlarge

Click to enlarge

Key collectors are interested in the variations among similar keys. If you would like to see more (much more) about the different types of J-38 keys in my collection click here for more than you ever wanted to know about J-38 keys.

EMail: scott(at)
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Creative Commons License
This work is licensed under a Creative Commons License.