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A MEDHURST DESIGNED SWITCHBOARD


4 Line Field Switchboard made by Siemens Bros. & Co. Ltd., England, 1916


Text by Bob Mills. Drawings by Ken Bushell



Tasmanian, F.W. Medhurst is well known for his design of telephone equipment mainly for use by the military. His designs were widely used by Australia, England and others during the First World War. One of his many designs was for Field Exchanges to connect his telephones together. At least two types of exchanges were known to have been made, 4 line and 6 line.


The switchboard is amazingly small, being only 120mm wide by 220mm high. It is 65mm thick measured overall and is provided with a ring so it can be hung on a wall. The small size is due, in part, to the fact that the switchboard works on one line plus earth return thus the size of parts are reduced considerably.

The layout of the switchboard is similar in many ways to a "standard" type pyramid switchboard (see figure 5). At the top is a large receiver used for extensions to call the switchboard. Across the bottom are 4 screw terminals for line wires designated "L1", "L2", "L3" and L4". There are then 4 "jacks" which, when plugged, connect each line in the idle state waiting for a call to be initiated. These jacks are designated "1R", "2R", "3R" and "4R". The second row of "jacks" from the bottom are used to connect each line to the operator's telephone both to allow speech and to send ring from the operators instrument to the distant extension. These jacks are designated "1C", "2C", "3C" and "4C". There are then 6 "jacks" representing every combination of extension to extension connection and, when plugged, allow any 2 extensions to converse. These jacks are designated "1-2", "2-3", "3-4", "1-3", "2-4" and "1-4". Two terminals are provided on the left, designated "C" for connection to the operator's telephone and one terminal on the right, designated "E" is the earth connection. Circuit diagram for the full switchboard is shown in figure 1 whilst circuit arrangements for each of the 3 switching situations are shown in figures 2 to 4.


Plugs for each of these jacks is a brass pin with a bakelite top similar to those used in resistance boxes. The simple plug arrangement is brought about by the simple circuit arrangements due to one leg to earth working. Each of these 4 plugs has a hole in the bakelite through which a thread is fed and this tied to eyelets each side of the receiver. This prevents the plugs from being lost in field operations where the switchboard may need to be operated in the dark.

One unusual feature of this switchboard is the large receiver or howler mounted at the top (see fig. 5). This receiver has a diaphragm 86mm in diameter and two coils in series, each of 100 ohms. The diaphragm is of steel and the "earpiece" of wood is held in place by 6 screws. This receiver is used to receive call in signals from extensions in the form of buzzer tone. The terminals on the switchboard were also of an unusual design to ensure reliable connections in the field (as shown in figure 5).

In operation, the calling extension would send tone down the line to the switchboard and this would be heard by the operator in the switchboard receiver (see figure 2). Each extension would send a different tone or series of tones as the operator needs this to determine which extension is calling. The operator removed the "R" plug from the extension's jack and places it in the "C" jack (figure 3). The operator can now converse with the calling extension. The operator then removes this "C" plug, leaves it hanging, and takes the plug for the called extension from the "R" jack and places it in the "C" jack.

The operator signals the called extension (by tone from the operator's telephone), tells them a call is waiting when they answer and then removes the plug from the "C" jack and places it in the jack representing connection between the two extensions (fig. 4).

There appears no way to indicate to the operator when the call is completed so it is assumed that the operator would need to monitor periodically by inserting the spare plug into one of the vacant "C" jacks.

Documentation shows that the design of this switchboard was from F.W. Medhurst however he appears not to have manufactured many of these units himself. It has been suggested that he did not make any instruments himself for sale but rather obtained stocks to sell from manufacturers licensed to make his products. This unit is stamped "Siemen's Bros. & Co. Ltd.", which means it was made in England. There is a serial number, 3874 and a date, 1916 (see fig. 5). It is known that GEC (England) also made switchboards similar to this one. There is also a 6 line switchboard designed along the same lines and this unit appears in figure 6.

This is a most unusual switchboard, not seen in many collections. Further information on its manufacture, use and the location of other units would be appreciated.



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