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Electricity in homes was initially restricted to illumination. Other domestic applications of electricity followed soon and necessitated development of wall sockets and matching plugs. The first two-pin plugs were designed in the mid 1880s. Concentric plugs appeared a couple of years thereafter.

Two types of concentric sockets and plugs have been designed:
(1) plugs with a central pin and concentric ring contact (see image 1); variants existed (see image 3);
(2) plugs that have a single pin with two contacts in succession (see image 2).

Concentric plugs are always polarized, in contrast to nearly all 2-pin plugs; a plus. Nevertheless their use has been restricted largely to special applications in which irreversibility matters (among other direct current), or voltage different from the standard domestic voltage. A disadvantage of single pin concentric plugs is their large size. Concentric plugs are rarely used anymore, except small jack plugs for ultra-low voltage and audio connections.

 

Plug with  central pin and concentric ring Sngle pin concentric plug
1
Plug with a central pin and a concentric ring contact. Typical dimensions are:
5 - 6 mm pin diameter and 8 - 9 mm spacing between pin and ring. Matching socket is shown in  image  no. 7.

2
Single pin plug. Typical contact diameters are 6 and 13 - 16 mm. Matching socket is shown in image no. 11. Sngle pin plug design could have be based on jack plugs, used from late 1870s as telephone switch board connectors.

 

Central pin connector

GEC concentric socket and plug

Gustav Binswanger patent GB 89516898, fig. 6
GEC Admiralty concentric plug and socket

3a, b
Concentric plugs and sockets that are based on a design by Gustav Binswanger, electrical engineer in London and founder of British General Electric Company (G.E.C.). Images 3a and 3b show two variants that date back to late 1890s - early 1900s; 3a is possibly a bit older design than 3b. Both variants were rated at 5A-250V.
Both have a large outer, ring shaped contact, but the central contact differs.
3a: socket has a central pin; consequently the plug must have a hollow contact (comparable to plug 3b).
3b: both socket and plug have ring shaped contacts.

 
3c Binswanger's patent application for "An improved Wall Socket and Plug for Electric Lighting and kindred Purposes" (British patent No. 16,898) dates back to June 10, 1985 and was accepted August 1, 1986.
Image 3c shows patent fig. 6*. Socket and plug have contact types similar to images 3d, 4-6 (Siemens) and 7 (Berger).
* redrawn and coloured to enhance visibility of structural elements.
 
3d Image shows a ca. 1910 design that was adopted by the British Admiralty.
GEC has made a specially adapted version for marine use. The socket had a gun-metal case, a separate cover for use when the plug has been removed and a lead scaling gland for lead-covered cables. Both versions were rated at 5A-250V.
 
Image sources:
3a: Electric Light Installations by David Salomonson, Volume II. Apparatus. 7th edition, 1894. Figure 174 (page 235).
3b: GEC 1904 catalog, page 423, catalog No. S 6638; also mentioned 1934 catalog, but no longer in the 1951 catalog.
3c: Patent GB189516898(A), see Espacenet link.
3d: GEC 1911 catalog, page 796; the marine version S 697 is shown on page 798.

 

Siemens concentric socket, inside Siemens concentric socket
Siemens concentric plug

4 - 6
Concentric socket and plug made by Siemens - Schuckertwerke in Berlin. Production started second half of 1920s and lasted until first half of 1950s. Rating: 10A - 250V. Schemes of socket and plug cross sections have been copied fro a 1938 Siemens catalog. The socket cross section shows an adequate steatite insulation between inner and contact ring. Image 4 shows that the insulation top part is replaced by a brownish Bakelite collar.
The Siemens catalog emphasizes the impossibility to exchange poles, a self-evident issue for DC battery chargers, but also important for laboratory equipment and for safe use of portable tools outdoors.   {RH}

 

Berger concentric 42V socket and plug

7
Concentric socket and plug, made by Walter Berger Electromechanische Ausrüstungen GmbH in Beeliz (Brandenburg, Germany). Rating: 10A - 42V*. Plug is not compatible with Siemens socket (no. 5), because Berger plug pin has a larger diameter (6.0 versus 5.0 mm). Outer rings have a comparable diameter (16.1 versus 16.0 mm).

* Low, "not dangerous" voltage local circuits were used in the German Democratic Republic (DDR), among others in wet rooms and at schools, hospitals and laboratories. Using concentric plugs avoid the risk of connecting 42V appliances to 220V. Production of 42V devices ended soon after the reunification of eastern and western parts of Germany.
PM (socket),  WN (plug) 

 

Classic 4A and 6A concentric plugs
Classic 6A concentric connector


Maehler & Kaege 10A-250V concentric plug

8a



 
8b
Single pin plugs with two brass contacts and handle made of Lignum vitae wood. Pugs have probably been made in the early 1900s mainly for more demanding, outdoor use. In Germany they were known as Garagenstecker or Zentralstecker. Shown examples and images in catalogs show they were often equipped with a watertight locking sleeve.
Versions existed that were rated at  4, 6, 10, 15 and 20 A. Wooden and ebonite plugs have been made until late 1920s.
None of the wooden examples in the collection have a manufacturers name or logo.   {WN}
 
6A concentric connector, matching with 6A plug shown in image 8a.
6A plug dimensions: pin tip diameter 6mm, length 23mm; proximal contact: diameter 10 mm length 24 mm.
9 Splash water resistant, concentric plug, rated at 10A - 250V. The housing has been made of a thermosetting urea-formaldehyde resin reinforced with cellulose.  Manufacturer: Maehler & Kaege in Ingelheim am Rhein. Dating: 1950s.   {WN}

 

WISKA socket for concentric plug
WISKA socket for concentric plug
WISKA concentric socket and plug

WISKA concentric plug
WISKA concentric plug and connector


Images 10 - 13 show a modern version of a classic type concentric socket and plug that is still used for marine applications. Material shown, made by WISKA (Kaltenkirchen, Germany), has been delivered by the Dutch WISKA representative Rometel Trade B.V.
 
10, 11
Concentric, polyamide, surface mount socket, rated at 10A - 50V. Image no. 10 shows the IP 56 watertight socket with closed screw cap; images no. 11 shows the concentric contacts.
12, 13
Socket and matching 10A - 50V, IP 56 plug. Pin and contact sizes are identical to 6A - 250V wooden plug shown in image 8a.
14 Despite ca. 100 years difference in age, the WISKA plug fits perfectly in a wooden 6A - 250V connector (image 8b). Even dimensions and screw thread of the brass locking collar have not been changed.

GEC single pin concentric plug and socket

15
 
General Electric Company 1951 catalog shows on page 235 ship's type connectors.
Nos S 899B and S 899A are respectively a single pin concentric cargo socket an plug.
The plug has a long Lignum vitae wood handle and resembles the old wooden plugs
shown in image 8a. Rating is not given, but it seems likely that  rating is comparable
to the WISKA devices, given the same target group.

 


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