Western Electric 1729 Point Contact Transistor (M1729, A1729, 2N25)  
Written by AnubisTTP on 2023-12-11  



Shortly after the invention of the transistor at Bell Laboratories in 1947, the engineers at Bell began the process of converting the transistor from a rickety science experiment into a useful and manufacturable device. In 1949 Bell began the production of two transistor part numbers derived from it's original 'Type A' production prototypes; the Western Electric 1729 linear transistor and 1698 switching transistor. These were point contact devices; a pair of spring loaded contacts were pressed against a crystal of germanium to form the semiconductor junction. Point contact transistors were an unreliable technology that required manual positioning of the contacts in order to work correctly, and all of the devices built around the original Type A cartridge package included a pair of round windows to allow for manual adjustment of the contact position. The window was covered by a sliding plastic tube when not in use. The tubular package of the 1729 and its relations was designed to be used with a special socket; a locking tab would hold the transistor in place once it was inserted. The 1729 was produced under several different part numbers over its lifespan, starting with the M1729 in 1949 and ending with the JEDEC 2N25 assignment in the mid 1950s. The example shown here is an A1729 and was produced in March 1954.

The original Type A transistor differed from the more usable 1729 in several ways, most notably in having its thick leads pre-bent at a right angle to the vertical axis that prevented a socket from being made for the device. The original Type A was only produced in small quantities to be sent to other companies for testing and experimentation. Point contact transistors in general were a short lived technology; when the more robust grown junction transistor arrived on the market in 1952, point contact devices were rendered obsolete. Outside of early hearing aids and a few telephone switching systems, point contact transistors saw almost no commercial use.


Western Electric 1729 Transistor
Western Electric 1729 point contact transistor. Later sold as the 2N25, the example shown here is mated with its matching socket.

Western Electric M1729 Transistor
The 1729 is a point contact device and has a side window to allow for manual adjustment of the point contacts.

Western Electric 1729 Transistor Box
Western Electric 1729 bulk transistor packaging. Each carton contains 20 transistors. 1729 transistors were also sold in smaller 'tube' style single device packages.

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