LearnTronics
History
Let There Be Light
Electricity leaves the laboratory and begins lighting the world
By the late nineteenth century, generators could provide useful electrical power continuously. One of the most obvious things to do with that power was also one of the most dramatic: make light.
The first electric light was an arc
In 1806, Humphry Davy demonstrated that an intense light could be produced by an electric arc between carbon rods. His batteries could demonstrate the effect, but they were not a practical source for lighting streets or buildings on a large scale.
That changed when useful generators arrived. By the 1870s, arc lamps were being developed into practical lighting systems. They were extremely bright and well suited to streets, factories, large halls, and other places where a powerful light was useful.
Arc lamps had disadvantages. Their carbon rods were gradually consumed, the spacing between the rods had to be regulated, and the light was far too intense for a small room or a lamp beside a chair.
Incandescence — make a conductor glow
A different approach was to pass current through a thin conductor until electrical resistance heated it so strongly that it glowed. This is incandescence.
The difficult part was keeping the hot material from rapidly burning away. Put the filament inside a glass bulb and remove most of the air, and oxidation can be greatly reduced.
- a thin resistive filament;
- electrical connections to each end;
- a glass envelope;
- and an evacuated or controlled atmosphere around the hot filament.
Joseph Swan and the practical lamp
The incandescent bulb was not the invention of one person in one moment. Joseph Swan in England experimented with carbon filaments in the 1850s and 1860s. Early vacuum pumps were not yet good enough for a durable practical lamp, but improved pumps allowed him to return to the problem in the 1870s.
Swan demonstrated a working incandescent lamp in 1879. His work was important, and in Britain his patent position later led the Edison interests to combine with him in the Edison & Swan United Electric Light Company, commonly called Ediswan.
Edison attacks the whole system
Thomas Edison was also working intensely on incandescent lighting in 1878 and 1879. The famous part of the story is the bulb, but the larger engineering problem was the lighting system around it.
A practical household lamp had to work at a useful distance from the generator, and many lamps needed to operate independently. A low-resistance lamp would demand large current and waste too much power in the distribution wiring. Edison therefore pursued a thin, high-resistance carbon filament that could produce useful light with much less current.
"make a wire glow."
It was:
"make a lamp whose electrical characteristics allow many lamps to be distributed and controlled as part of a practical power system."
The Menlo Park laboratory tested many carbonized materials. Carbonized thread and paper were used in important experiments and demonstrations, and bamboo later became a commercial filament material. By the end of 1879, the laboratory could demonstrate a practical incandescent lighting system.
The bulb needs an electrical industry around it
Electric lighting required far more than glass bulbs. A complete system needed generators, feeders and branch wiring, switches, sockets, fuses, meters, insulation, and a way to distribute power safely to many customers.
On September 4, 1882, Edison's Pearl Street central station in New York began operation. A generating station could now supply electric light to customers through a distribution network. Electricity was becoming a public utility rather than merely a laboratory phenomenon.
A light bulb is also an electrical load
Measurement trail 16 — power in an incandescent lamp
An incandescent lamp turns electrical power mainly into heat, with part of that energy leaving as visible light. The familiar power relationships let us estimate what the lamp demands from the circuit.
A 60 watt, 120 volt lamp
At its rated operating condition, the current is approximately:
Its hot operating resistance is approximately:
The filament resistance is not constant. A cold incandescent filament has substantially lower resistance than it does when white-hot, which is why the turn-on current can be much larger than the normal operating current.
Then the light bulb did something unexpected
While working with incandescent lamps, Edison encountered another effect inside the evacuated bulb. Material leaving the hot filament could darken the glass. In experiments intended to understand and reduce such problems, an additional metal electrode was placed inside a lamp.
In 1883 Edison found that current could pass through the evacuated space between the hot filament and the extra electrode under the proper polarity. The effect became known as the Edison effect. Edison patented an electrical indicator using it, but he did not develop it into the electronic device that would later become so important.
The glass bulb has nevertheless given us something important to remember: a heated electrode inside an evacuated envelope can move charge across empty space.
Lighting also forces power systems to improve
A useful lamp is only as useful as the electrical system feeding it. By the early 1880s, engineers were building better generators and experimenting with alternating-current systems that could change voltage with a transformer.
That device — the transformer — will make alternating current far more useful for distributing power over distance. The commercial arguments over DC and AC are historically interesting, but for LearnTronics the more important question is what the devices themselves actually do.
alternator + transformer → practical AC distribution
heated filament in vacuum → a clue that will later become the tube diode