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The idea in its more general form is to have a device which measures how much money or other resource is being used at any time, and as a consequence, you can adjust your behaviour with that in mind.
It's easier to keep expensive phone calls brief if you know how much money it's costing per minute.
A good example of the Econometer is a "miles per gallon meter" in a car. This gives a continuous read-out of how many miles per gallon are being achieved at that moment, and so more economical driving habits are developed. This is also mentioned on the review of car fuel saving
One obvious application of an Econometer is in the household electrics. If you had a readout of how much electricity you were using right now (and how much it's costing) you'd probably go round and turn off a few things to save money.
Money spent on electricity per time is directly proportional to the power being used at that moment. It's effectively a power meter. The power is also directly proportional to the current. So, the next question on building such a device is: How to measure the current going through a household electric system? Putting a huge ammeter in series with it may sound easy, but in practice it would be inconvenient to install as it would require unhooking those big cables that go into the main fuse box. In practice, there is a much more neat and safe way to achieve the same result.
The idea of an electric economy meter relies on measuring the current, but if you put a big ammeter in it would have a big shunt in it. So, instead of doing that, let's use the copper cables already installed as a shunt! These have a low (non-zero) resistance, and would therefore have a voltage across them. So, by connecting up some thin wires to the two ends of a heavy copper wire, through a diode, to a galvanometer, the current should be proportional to the power being used. (This is also much safer than doing the heavy engineering work!)
A further refinement to the idea is, instead of using the wires as a shunt, a reading could be taken by putting a small induction coil near the wires (as it's AC), rectifying the result, and putting that into the galvanometer. Update: You can actually buy a device that does this, right away. Look for "CLAMP METER" on Maplin
In any of these things, the meter needs to be calibrated, but that can easily be achieved by comparing the reading with and without a test load such as a 1Kw electric fire.
If you try any of this, make sure you do it safely! Don't work on it LIVE and don't connect things across the supply!
After installing an Econometer, you may be surprised at how much electricity is being used up by devices you'd forgotten about. Standby current on televisions, lights that are left on, and power supplies that were plugged in years ago and are still there.
The result: Saving electricity and saving money.
Update: You can now buy a device ready-made to do this! One of these is called the Wattson Home Energy Meter and is available from Drink Stuff would you believe?! direct link (gone)
Further update: In late 2008, econometers or "energy monitors" were made available to the public as a free bonus on some of the special energy plans by E.ON! How about that?! Also, Southern Electric have started doing this now, giving away econometers.
No page like this is complete without pointing out you can also save money on your electricity by checking you're getting the best deal by comparing different energy companies, either by doing it yourself or by visiting uSwitch or Energylinx
On 2008/10/20 I saw on the television programme "Despatches" Allan Asher of EnergyWatch saying something to the effect of "Wouldn't it be good if everyone had a SMART METER". Well, the Econometer is in effect a smart meter, at least to the extent that it tells you how much electricity or fuel you are using, in real time.
Safety note: Because of INDUCTION, it is entirely practicable for amateur electronics enthusiasts to implement an econometer next to their electricity meter without connecting things to the dangerous live supply. For example, Southern Electric's energy monitor works by having an inductive sensor in the cupboard under the stairs, which then sends a signal by wi-fi, to the econometer which can be anywhere else in the house!