Conversation Topics: What is a Watt?
We often hear about the megawatt hours of electricity generated to keep our lights and air conditioners on, but we give little thought to the origin of the term "watt." During these steamy days of August, it's appropriate to dabble a few minutes in the story of James Watt from Scotland to understand why a unit of electricity is a named a "watt" after a man who never dabbled in electricity.

Perhaps we should start by noting that the practical uses of electricity were invented by others who worked well after his death. Watt died in 1819, but Faraday didn't invent the direct current electric motor until the 1830's. Edison's electric light followed much later in the 1870's. And Tesla, who invented the alternating current motor like the one found in your air conditioner, didn't conceive of it until the 1880's. So what did James Watt do to earn this distinction?

Watt had a number of inventions, including the distance measuring telescope often seen employed by highway construction crews (one at a tripod, a second worker holding a long pole) to mark distances accurately. But his real claim to fame is his improvements to the steam engine, improvements that were so dramatic (around four times the efficiency) that they made practical the use of steam engines in factories and for transportation. The secret to Watt's inventions was his realization that you need to keep the cold side cold and the hot side hot, an idea that McDonald's used just a few years ago with the McDLT.

The physics of a steam engine are simple. If you heat water to its boiling point, water turns to steam, and as steam, water takes up a lot more volume than it does in its liquid form. That's why a tea kettle whistles: the water is turning to steam and there just isn't enough room in the kettle for it and the water to share; so, out the spout it goes, whistling all the way.

That expansive force, if contained rather than allowed to escape freely into the open air, can be used to push things, and in a steam engine, that thing is a piston, a moveable top to an otherwise sealed and rigid cylinder. The expanding force of the steam causes the piston to move (since the other walls of the cylinder are immovable), and the piston's movement can be translated into motion for productive purposes. A simple enough idea, but unfortunately, that steam is not very powerful and builds up very slowly, so early steam engines actually used the opposite effect: if you cool steam (which you can do rapidly), it contracts and creates a vacuum. That vacuum was used to 'pull' the piston into the cylinder (rather than push it out) and thus drive the machines.

What Watt observed was that steam engines would heat water in their boiler, pipe it into the cylinder, and then cool it in place to cause the vacuum. But this meant that the piston and cylinder were then also cool and so would begin cooling the new hot steam as it was let into the piston for the next cycle. This cooling of the steam reduced its expanding power and thus, its ability to contract later when cooled.

Anticipating the McDLT, Watt's idea was to separate the hot cylinder and piston from the cooling process through the introduction of a separate "condenser." That is, after the hot steam had filled.the cylinder, valves open and shut so that the boiler is mechanically disconnected and the condenser connected to the cylinder. The condenser provides the vacuum to the piston without also significantly cooling the cylinder and piston. Once the piston is pulled in, the valves switch the connections back to the original way, and only the steam that is in the condenser is cooled. This way, much of the power of the steam isn't lost before it's put to productive use.

So now that we're all bathing in steam, how does that answer why scientists named the watt after a man of steam? Because if you look inside most power generating plants, you will find that the electricity is created by a generator (which is a motor acting in reverse -- changing motion into energy). What turns the generator? In some cases it's turned by either the wind or water (hydroelectric); but most commonly, in a gas, coal, or nuclear power plant, the generator is turned by a steam-driven turbine, the modern replacement for the steam engines that powered the first power plants. In this way, Watt's improvements in the steam engine, the power source for the early electrical generators, made possible the widespread and high volume generation of electricity.

James Watt built a better steam engine, but the day of the steam engine has past. Even though today's methods of generating electricity are also changed (steam turbines instead of steam engines), their output is still the same -- electricity, and so it is fitting that the unit of electricity still bears his name. This is much like the fact that the unit of power of motors and engines of all kinds are still rated in "horsepower" -- another Watt legacy. But that story we'll tell another day.

Source: Encyclopedia Britannica

Updated September 30, 2003 - go to our home or life advice page