The board as a machine
A manual switchboard is a surprisingly simple machine. It has no moving parts beyond the operator’s hands. Its function is to bring the line of each subscriber to a point where a person can reach it, and to provide a way to join any two lines together. The parts that did this work stayed fairly consistent from the 1880s to the mid-twentieth century.
The main parts
Jacks. A jack is a socket, and a switchboard has hundreds or thousands of them in rows. Each subscriber’s line ended in at least one jack. In a multiple board, the same line appeared as a jack in front of every operator, so any operator could reach it. A jack for answering was usually accompanied by a signal.
Plugs and cords. The plug is a metal connector at the end of a flexible cord. The cords come in pairs. One cord of the pair is used to answer a caller, and the other to reach the person called. The two cords are joined inside the switchboard by a circuit that carries the conversation and supplies power. Cords were weighted underneath the board, so they retracted when the plug was pulled out. Power for the lamps and the talking circuit came from the exchange battery, usually a lead-acid battery bank.
Signals. Early boards used a small shutter that dropped when a subscriber cranked the telephone’s magneto, showing the operator which line was calling. Later boards used small lamps, which lit when a caller lifted the receiver. Each cord pair had its own supervisory lamps, which told the operator when the parties hung up.
Keys. Each cord pair had switches, called keys, that the operator flipped to speak to the caller, to ring the called line, or to listen. The operator wore a headset with a receiver at the ear and a transmitter on a chest piece or arm, which left both hands free for plugs.
Tip, ring and sleeve
Telephone plugs had three contact areas, called tip, ring and sleeve, named for the parts of the plug. The tip and ring carried the two wires of the voice circuit. The sleeve was used for signaling, such as showing whether a line was in use. The same plug style is familiar from guitar amplifiers and headphones. The terms “tip” and “ring” are still used by telephone technicians, and “tip, ring, sleeve” connectors also survive in audio equipment and headphone sockets.
How a call was connected
In broad terms, a typical call at a common-battery board in the early twentieth century went like this:
- The caller lifted the receiver. A lamp lit above the caller’s jack.
- The operator picked up the answering cord, inserted the plug in the caller’s jack, and flipped the key to speak. She asked for the number or name.
- The operator located the jack of the called line on the board. Before connecting, she touched the tip of the second plug to the sleeve of the jack to test whether it was busy. In older systems, a click in her headset told her that the line was in use, and silence told her it was free.
- If free, she inserted the plug and pressed the ringing key, which sent current to ring the called telephone.
- When the called party answered, the lamp for the second cord went out. The operator left the cord pair connected and attended to the next call, watching the supervisory lamps.
- When both parties hung up, the lamps lit again, and the operator pulled the plugs, which freed the lines.
A skilled operator could do this in a few seconds. The speed and accuracy of the process were a point of pride.
Calls to other exchanges
For calls beyond the local exchange, the operator used a trunk line to another exchange or a different board. In larger cities, calling and called lines might be handled on separate boards, with trunks between them. Long-distance calls required more time, with operators at several points in the route working together to establish the connection.
Different kinds of boards
Not all boards were the same size. Small-town boards might have a few dozen jacks and one operator, who covered the local business and the night alarm. Business offices and hotels used smaller boards, called private branch exchanges or PBXs, to connect their own internal telephones with each other and to outside lines. Some of those small cord boards lasted into the second half of the twentieth century.
Materials and design
Switchboards were built from wood, brass and copper with mechanical care. Many boards had a polished cabinet and a cord shelf, with the board tilted to a comfortable working angle. Designers considered reach and posture carefully, and the arrangement of jacks was designed to minimize stretching. Chairs, headsets and lighting were also adjusted to the work.
What remains
Patch cords were also used elsewhere: studios for radio broadcasting used patch panels, and early electronic computers were set up partly by plugging cables. The switchboard has become an icon of early telephone service, and people who see old boards in museums often find them surprisingly intuitive. The same ideas, a central point, labeled connections and temporary links, live on in modern equipment, where software makes the connections that plugs once did.