Switching Basics and More 49Experiment 6: Very Simple SwitchingBAckground
Early switching systems
Switches seem to be such a fundamental feature of our world, and their concept
is so simple that it’s easy to forget that they went through a gradual process of
development and refinement. Primitive knife switches were quite adequate for
pioneers of electricity who simply wanted to connect and disconnect electric-
ity to some apparatus in a laboratory, but a more sophisticated approach was
needed when telephone systems began to proliferate. Typically, an operator at
a “switchboard” needed a way to connect any pair of 10,000 lines on the board.
How could it be done?
In 1878, Charles E. Scribner (Figure 2-34) developed the “jack-knife switch,” so
called because the part of it that the operator held looked like the handle of a
jackknife. Protruding from it was a plug, and when the plug was pushed into a
socket, it made contact inside the socket. The socket, in fact, was the switch.
Figure 2-34. Charles E. Scribner invented the “jack-knife switch” to satisfy the
switching needs of telephone systems in the late 1800s. Today’s audio jacks still
work on the same basis.*
Audio connectors on guitars and amplifiers still work on the same principle, and
when we speak of them as being “jacks,” the term dates back to Scribner’s inven-
tion. Switch contacts still exist inside a jack socket.
Today, of course, telephone switchboards have become as rare as telephone
operators. First they were replaced with relays—electrically operated switches,
which I’ll talk about later in this chapter. And then the relays were superceded
by transistors, which made everything happen without any moving parts. Be-
fore the end of this chapter, you’ll be switching current using transistors.
- The photo on which this drawing is based first appeared in The History of the Telephone by
Herbert Newton Casson in 1910 (Chicago: A. C. McClurg & Co.).