This switchboard has a capacity for 20 telephone sets. (If necessary 24 sets can be connected.) This type is therefore suitable for such offices or factories where the number of sets within the following years is not likely to exceed about 20.
Five simultaneous connections are provided for.
The apparatus is mounted in a case of polished oak.

Dimensions of case:
Height: 920 mm.
Width: 386 mm.
Depth: 200 mm.

Either one or two storage batteries are required for a switchboard and arrangements for charging can be provided suited to local conditions. Prices of batteries and arrangements for charging on application.
The telephone sets (DB 50 and DB 250) which are used with this switchboard are provided with a dial for calling and a press button for signalling. The dial is provided with 10 numbered holes. By means of the dial the desired connection is completed in the following manner:
If telephone set No. 13 is required, the micro telephone of the calling set is first lifted and the tip of a finger then placed in the hole marked (I) and the dial is turned in a clockwise direction until the finger strikes a stop, whereupon the finger is removed and the dial returns to normal under the influence of a spring. The same procedure is then repeated with the hole marked (3). When the dial again returns to normal the connection is accomplished and a signal is given by means of the press button. If the signal is heard in the calling receiver, the desired number is free, but if no sound is heard, the number is busy.
When the conversation is finished and the two micro telephones replaced, the two lines are at once automatically disconnected. Should certain lines connected to this switchboard require communication with the public telephone exchange, telephone sets on these lines must be of the DC 400 type.

The principal advantages of the automatic telephone switch OL 20.

1. It is always ready for use, by day or by night.
2. It is independent of an operator.
3. The connections are promptly and easily made and cut.
4. The conversation is secret.
5. Little space is required.
Automatic telephone switchboard for 20 lines.

Taken from the L. M. Ericsson Telephone Catalogue No. 1921

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Last revised: August 15, 2002