Museum of Plugs and Sockets logo, small Classic flat blade
plugs and sockets
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Oder material can still be found when old homes are renovated. Examples of classic models of flat blade sockets, plugs and adapters are shown below. Two other objects, related to sockets have been added to the page: (1) radio outlets and (2) knob and tube wiring.

The origin of US flat blade plugs is described on a separate page.


Classic US parallel-tandem sockets Monolite quintet receptacle General Electric 4-plug outlet

1, 2
Two examples of classic non-grounded sockets with slots for plugs with two parallel blades and  for plugs that have blades in tandem orientation. Both types of plugs have been designed by Harvey Hubbell. The tandem blade configuration was patented in 1904, the parallel version eight years later. The page on origin of US plugs gives detailed information.
For many years both types were available, but finally the parallel orientation became the standard US 15A - 125V plug.
The coexistence of parallel and tandem blade plugs necessitated the construction of wall sockets that could accept both types.

Manufacturers: General Electric (no. 1), and Hubbell (no. 2). Sockets
became available during renovation of a 1937 home in California.   {BN}
3 Monolite Quintet socket, rated at 15A-125V / 10A-250V (see no. 7 for explanation of two different ratings). The socket is polarized. Slots at right are 1.6 mm wider than corresponding slots at left (8.72 versus 7.12 mm), see inset.
Brand name: Monowatt Electric Corporation, New York, Providence, Chicago. Socket dates back to late 1930s, when Monowatt was a division of General Electric.   {RP}

Socket nos. 3 and 4 came in their original carton. They are shown on a separate page.

4 4-plug socket, rated at 15A-125V. The dual - duplex design probably dates back to the mid 1950s.
Manufacturer: General Electric (wiring device department, Providence).

Obsolete type-A plug and connector Obsolete type-A plug and connector Obsolete 2-pin plug in T-configuration Comarison of T-slot pluf and NENA 2-20P

5, 6 Standard flat blade plug and connector made of ebonite, a vulcanized natural rubber. A relative high sulfur content and prolonged times of vulcanizing results in a solid product. Manufacturer: Arrow Electrical Wiring Devices. Dating:  ca. 1930s.  {ChR}

As many older North American devices the connector shows two different amperage and voltage ratings:
10A - 250V
15A - 125V

This has to do with a peculiarity of the National Electrical Code from 1923 to the 1950s. Originally, sockets were rated at 10A 250V, because the NEC limited lighting circuits to 10 amperes. In 1923, the code changed to allow lighting circuits to be fused at 15 amperes; however, the old rule still applied to circuits over 125 volts.
7, 8
Obsolete plug with two flat blades in T-configuration, rated at 10A - 250V. The U.S. company Bryant* has made T-slot sockets for this type of plugs. They were rated at 15A - 125V and 10A - 250V. T-slot plugs do not fit in socket no. 2 and are incompatible with NEMA 2-20 sockets, because of different pin spacing (see image no. 8). Manufacturer of the shown plug is not indicated; it could have been made in Australia, since the type has been used also in Australia and New-Zealand in the 1930s - 1950s.   {ChR}
* Bryant Electric Company in Bridgeport, CT (1888-1988, thereafter part of Hubbell Inc.).

pre-NEMA 15A-125V / 10A-250V receptacle pre-NEMA 15A-125V / 10A-250V receptacle
Classic NEMA 1-15R socket with switch Earth key for NEMA 1-15 socket

Non-NEMA duplex socket with angled slots for line (hot) and neutral (w). It is rated at 15A-125V and 10A-250V.
The two sockets are each other's mirror image, an unusual orientation, but the top and bottom angled slots are correctly
(crisscrossed) wired. Manufacturer: Hubbell (model 7051)
10 Hubbell 10A-250V, 15A-125V plug that fits in socket no. 9. The type of plug with angled power pins is described in more detail on the origin of US flat blade plugs page. The plug has an Underwriters Laboratories* certification mark.  * U.S. testing facility and developer of safety standards. Dating: probably 1950s.
11 Classic model of a two-blade (non-grounding) 120V, 15A socket, with switch. The switch at left is an additional light switch.
Nowadays only grounded sockets are allowed, but non-grounded plugs (NEMA 1-15P) are still permitted for appliances that don't require an equipment ground connection.
Manufacturer: Leviton. US patent no. 2704832 was granted on March 11, 1955.
12 US ground adapter for 15A - 125V two pole socket. The grounding clip of the adapter (green arrow) is connected to earth via the brass screw that is used to attach the wall plate to the body of the socket (which have to be grounded !). These adapters are also known as cheater plugs. In 2003 they were still available in some hardware stores.

TAP-A-LINE outlet with plugs
Lamp holder with US flat blade two-pole outlets
US lampholder adapters

13 Tap-A-Line outlet with elongated slots to receive the blades of multiple appliance plugs. Details are given on a separate page.
14 Pull chain medium size* light bulb socket with two non-grounded outlets. Only the lamp is switched on/off by the pull chain; outlets are continuously active.
* medium refers to the diameter of the Edison screw base; medium = 26 mm (U.S.), or 27 mm (Europe). Size are also indicated as E26 and E27. The adapter has been made in Colombia
15 Two medium size light bulb adapters. Left: screw base to 2-pole outlet*. Right: non-grounded 15A, 125V plug to medium light bulb socket. Manufacturer: Leviton.

* screw base outlets date back to a 1904 Hubbell patent, see origin of US flat blade plugs for details.
  An original Hubbell separable attachment plug and other examples of screw based outlets are shown on a separate page.

US plugs with automatic wiring facility
NEMA 1-15R made by Busch-Jaeger, Gemany; accepts also Europlugs Terko - NEMA 5-15 adapter

16 Both General Electric Company and Academy Electrical Products Co. (later part of Eagle Electric and therafter Cooper Industries) have designed plugs and connectors with an automatic wiring method; tools are not needed. Details about three automatic wiring methods are given on a separate page. These plugs date back to the early 1950s; some are still available.

↓ note

Nos. 17 and 18 have been made for the U.S. Army in Germany. They were for sale in Germany in the 1950s to ca. '80s.
17 Not earthed duplex socket with slots in parallel and in tandem position, rated at 15A - 250V (see also no. 2). Slots accept also not earthed continental European 2-pin plugs with 4.0 or 4.8 mm pins. Original wall plate is missing. Manufacturer: Busch-Jaeger in Lüdenscheid, Germany. The socket has a type of Busch-Jaeger logo that has been used from 1951 until 1979.   {FSE}
Adapter plug for NEMA 5-15 plugs (image left) that fits in Terko* sockets (image right). Note that NEMA 5-15 is a 120 Volt standard whereas the German Terko system is rated at 250 Volt. The adapter does not have a transformer.
The adapter has a MPAD molding mark. No. 76 refers to the company that has produced the cast: Bezet-Werk, Herman Buchholz GmbH in Berlin-Lichtenrade. No. 131 stand for an urea-formaldehyde resin with cellulose as filling agent. It is not clear whether Bezet-Work also has assembled and sold the adapters.
* See Terko page for details.


Radio outlets

In many homes, build in the 1920s to '40s radio antenna coils were installed at the attic. A wire connected the antenna to a grounded radio outlets in rooms downstairs.
When home antenna coils fell into disuse, radio outlets were often put out of sight by wall paper or layers of paint (see image No. 19).

Although radio plugs had a pin configuration that never has been used for 120 or 240 Volt power connections, it is confusing that often radio outlets were part of a dual socket; the other half was a standard 15A-120V outlet. 

US radio outlets

Radio outlet made by Hart & Hegeman in Hartford, Conn.
H&H merged in 1928 with the Arrow Electrical Company to become the Arrow-Hart & Hegeman Electrical Co. Photo has been sent by Joshua Hodges.
The dual socket has a radio and NEMA 1-15 socket.


Radio outlet designed by Victor R. Despard. Patent application was filed in 1932 by Pass & Seymour Inc. in Syracuse, N.Y. Outlet has been made also by Bryant Electrical Corporation in Bridgeport, CT.


Knob and Tube wiring (K&T)

Knob and tube wiring was an early standardized method of electrical wiring. It used individual wires supported by porcelain insulators (knobs) and run inside porcelain cylinders (tubes) when going through wood beams. See image nos 21 - 24.
Until the mid-1930s, knob-and-tube wiring was installed in many U.S. homes and it can still be found in some older houses.

Whether or not K&T has to be replaced is discussed on several web sites. See for example:
httpsss:// and

In short: knob and tube wiring, properly installed and in good condition is both reliable and safe, but it has no ground wire and is not appropriate for electrical equipment that requires grounding.
K&T is not permitted in new constructions in the US, except in a few industrial and agricultural related situations specifically listed in the US National Electrical Code.

Knob and Tube wiring (1) Knob and Tube wiring (2) Knob and Tube wiring (3) Knob and Tube wiring (4)

21, 22 Porcelain knob insulator. This example consists of two pieces and has on each side of the nail a pass-through groove to guide the wire. Knobs are nailed into wall studs and floor joists. Usually hot and neutral wires are separated from one another, which allows the wires to readily dissipate heat into free air. Therefore wires are capable of carrying higher currents than the same conductors in close proximity. Another type of insulator (not shown) consists of a single piece of porcelain. The wire loops in a circular groove running around the circumference. Also block-shaped ceramic cleats are occasionally used. {BN}
23 Porcelain tube, used to keep wires from coming into contact with, or being compressed by wood framing parts of a building. {BN}
24 K&T wiring inside a 1930 Pittsburgh house. Part of a photo taken in 2006 by Laura Scudder and shown at the Wikipedia page about knob and tube wiring.


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