Discover the Ingenious Step-By-Step Switching System

Discover the Ingenious Step-By-Step Switching System

Table of Contents

I. Introduction

II. The Bell System and Automatic Dialing

III. The Investment in Switching Equipment

IV. Step-by-Step Switching

A. The Line Finder

B. The First Selector

C. The Second Selector

D. The Third Selector

E. The Connector

V. Switch Operation in Detail

A. Mechanical Parts of the Selector

B. Electrical Parts of Switch Operation

C. The Importance of Maintenance

VI. Conclusion

The Bell System and Automatic Dialing

The Bell System was a system of companies that provided telephone service throughout the United States for over 100 years. One of the key innovations of the Bell System was the introduction of automatic dialing equipment that made it possible for customers to dial their calls accurately and quickly. This equipment was carefully engineered and manufactured to close tolerances to ensure its reliability and accuracy.

The Investment in Switching Equipment

The switching equipment that allowed for automatic dialing was a significant investment for the Bell System. In one particular city, the equipment to Take Care of 20,000 customers represented an investment of about two million dollars. Proper installation and adjustment of the equipment was essential for its proper operation, and the telephone men who knew their circuits and equipment thoroughly played a critical role in maintaining the system's reliability.

Step-by-Step Switching

The switching equipment used by the Bell System was called step-by-step because each call progressed through the system one step at a time, as each digit was dialed. The system was composed of five switches called selectors that operated in a step-by-step manner to connect the caller to their intended party.

A. The Line Finder

The first switch, called the line finder, operated when the calling customer raised their receiver. The line finder would hunt the calling line and then connect to the calling number's terminal.

B. The First Selector

The line finder was connected permanently to the first selector, which returned the dial tone to the calling customer to indicate that the equipment was ready to receive the first digit. When the first digit was dialed, the first selector stepped up to the third level and automatically selected an idle circuit to connect to the next selector.

C. The Second Selector

The Second selector operated when the second digit was dialed. It stepped up to the fifth level as it responded to the second train of dial pulses and cut in automatically to extend the circuit to the third selector.

D. The Third Selector

The third selector operated in the same manner as the second selector, extending the circuit to the connector switch when the next digit was dialed.

E. The Connector

The connector switch operated somewhat differently from the selectors. It responded to the dial pulses but did not cut in until the called party's terminal was selected. The connector waited for the final digit to be dialed and then rotated to the appropriate terminal to send ringing Current out on the called line until the receiver was raised.

Switch Operation in Detail

The step-by-step switches that comprised the Bell System's automatic dialing equipment were complex mechanical and electrical systems that had to operate with a minimum of wear and perform satisfactorily in spite of minor variations in customers' dial speeds or their distance from the central office. The Bell System's engineers carefully designed and tested these switches to ensure their long and accurate service.

A. Mechanical Parts of the Selector

The mechanical parts of the selector included five relays, designated A, B, C, D, and E, magnets, and mechanical linkage. The shaft of the selector was moved vertically by the magnets and linkage and also rotated horizontally. A release magnet and armature released the shaft when the call was completed.

B. Electrical Parts of Switch Operation

The electrical parts of switch operation were more complex, but high-speed photographs helped engineers design the equipment for long and accurate service. Minor adjustments were required to ensure the switches operated with a minimum of wear, and maintenance personnel played a critical role in proper adjustment and care of the equipment.

C. The Importance of Maintenance

Good design, a well-made product, and accurate adjustment meant good service for the Bell System's customers. The Bell System's engineers carefully developed maintenance procedures so that switches would Continue to operate reliably and accurately, even under heavy usage.

Conclusion

The Bell System's automatic dialing equipment was a significant innovation that allowed customers to dial their calls accurately and quickly. The system's complexity required careful engineering, manufacturing, installation, and maintenance to ensure its reliability and accuracy. The step-by-step switching system used by the Bell System was a marvel of engineering that relied on both mechanical and electrical systems to connect callers to their intended parties quickly and accurately.

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