How a Dial Phone Works (1/2)

Part 1: General

Since using all my early life dial phones but never actually studied how do they work .. now it is the time. In this article I open a LM Ericsson rotary telephone and figure out how it works and how does the automatic central connect the caller to called.

A dial phone opened with its basic parts visible: base plat, PCB (Printed Circuit Board), ringer, dial plate, cover, handset with microphone and receiver capsules in it, plug and cables.

The idea of an automatic telephone is old, over 100 years now. Each telephone is connected using 2 wires to the central (3rd wire being the Earth). In manual central simply two phones are connected together and also a DC power source is added to the line so that the carbon microphones work (they need direct current to transform the voice vibrations to electrical current vibrations).

LM Ericson dial telephone from 1970's-80's. Model (H15662) DLG/DLN 012 024

The plug shows 3 pins in the wall connector but only 2 of them are used for the connection. The third pin carries the additional R+ signal for an external additional ringer. The 3rd pin is there only to hold the plug better in place than if it just had 2 pins. The 2 pins required for the connection carry the La and Lb symmetrical signals (against the Earth potential which is usually not connected at all).


TELEPHONE'S MECHANISMS

In an old electro-mechanical telephone the hook is very a important element. The hook operates a switch with 2 states (up and down).

1. When the hook is down the telephone is only a ringer. The talk circuits are disconnected. At this state the telephone is ready to receive an 20-25 Hz AC signal which will sound the bell or the ringer inside the phone. It is impossible to listen a phone with the hook down since the microphone is disconnected.
2. When the handset is lifted up the hook jumps up and the telephone's talk circuit is connected to the line and the bell is now disconnected. In this state the telephone is ready to receive or transmit sound or the dial plate could also be used. When the hook is up and the dial plate is rotated the sound circuit is shunted and the shunt opens to send impulses to the line. These impulses are used by the central to connect the caller's line to the called line (according to the number dialed). The impulses control the central's automatic electro-mechanical switches.

The following video shows how a dialer works.

This video shows how the 2 switches in a dial plate work. The first one closes (and/or opens) when the dial is turned (to break the sound line etc.) and the other one sends impulses when the dial returns (to inform the number dialed to the central).

In the telephone central there is one or more Strowger switches (named after its inventor). The following video illustrates the operation of the dialer and the Strowger switch. (A single Strowger switch can typically connect to 100 lines [10 vertical levels and 10 rotary positions plus nil]).

Video about how the dialer activates the telephone switch.

The following video shows how several switches work together to form longer numbers and allow more subscribers.

Video: "AT&T Archives: The Step-By-Step Switch"

The following diagram describes the step-by-step switch shown in the above video.

Step-by-Step Switch operation (5 digit numbers used)

On the left of the diagram local telephones are connected to a local line finder and line connector. In this example a local caller calls another local telephone. The last selector is always a connector and it takes the final 2 digits of the number called. In this example 5 digit numbers are used so there are 3 additional selectors. The 3 intermediate selectors only use a single number for the vertical movement. The rotational movement is done automatically by a devices which assigns the next free trunk line for the caller (or signals busy if none available).

Strowger switch principle 

The line finder differs from the selectors and connector so that it hunts automatically the caller's phone line when ever the handset is lifted off the hook. Since a single line finder can only connect a single call there must be as many line finders as there are concurrent phone calls allowed in this central. Notice that the selectors do not have to be located in the same central and that the phone calls can advance outside the local central.

Minimal 100 line system with 3 concurrent calls

The above diagram shows a minimal 100 line (100 subscribers) system where 3 concurrent phone calls can be delivered. In this system you need 3 sets of line finders and line connectors each for 100 phone lines. If one line finder is busy the next in the stack will handle the call. This system would need 6 Strowger switches all together.

(Part 2 of this article will go more in the details.)


REFERENCES

/1/ Wikipedia

/2/ KEMPSTER B. MILLER, M.E,
      "TELEPHONE THEORY AND PRACTICE,
      AUTOMATIC SWITCHING AND AUXILIARY EQUIPMENT",

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