Page 44 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Semiconductors: Diodes and Transistors 25

The relay is a digital component, L O - - - -
because it is either ON or OFF. By
connecting relays together in different
When the coil is energized,
ways, it’s possible to create all sorts of its electronic field pulls
things. Perhaps the most ambitious use the switch closed
of relays was to build gigantic electro- XW
mechanical computers, such as the Figure 4-2. The electromechanical relay
Harvard Mark 1. Constructed between
1939 and 1944, the Harvard Mark 1 was 50 feet long, 8 feet tall, and contained
over 750,000 individual components.
The problem with relays (especially the types that were around in the early
days) is that they can only switch a limited number of times a second. This
severely limits the performance of a relay-based computer. For example, the
Harvard Mark 1 took approximately six seconds to multiply two numbers
together, so engineers were interested in anything that could switch faster. . .

The First Vacuum Tubes
In 1879, the legendary American inventor Thomas Alva Edison publicly
demonstrated his incandescent electric light bulb for the first time. This is the
way it worked. A filament was mounted inside a glass bulb. Then all the air
was sucked out, leaving a vacuum. When electricity was passed through the
filament, it began to glow brightly (the vacuum stopped it from bursting into
flames).
A few years later in 1883, one of Edison’s assistants discovered that he
could detect electrons flowing through the vacuum from the lighted filament
to a metal plate mounted inside the bulb. Unfortunately, Edison didn’t develop
this so-called Edison Effect any further. In fact, it wasn’t until 1904 that the
English physicist Sir John Ambrose Fleming used this phenomenon to create
the first vacuum tube.’ This device, known as a diode, had two terminals and
conducted electricity in only one direction (a feat that isn’t as easy to achieve
as you might think).

1 Vacuum tubes are known as valves in England. This is based on the fact that they can be used
to control the flow of electricity, similar in concept to the way in which their mechanical
namesakes are used to control the flow of fluids.