How, specifically, does an electric meter measure wattage?

Did he specify what meter he'll be using?

About the only way I can think of doing this is to try and exploit the accuracy limitations that all
electronic electricity meters have.
Most modern electronic electricity meters are Class 1 meters which have a variety of accuracy specs
depending on how much energy they have measured.
A modern utility meter is Class 1 with a very tight requirement to be accurate to +/- 1% down to 5% of load.
If the Meter has a maximum current rating of 100 A, then the meter will be accurate within 1% down to
50 ma , so anything below this means that the meters accuracy is not defined.
If the supply voltage is 240 V , then a load of less than 12 watts will be below the meters minimum accuracy standard
and whilst the meter may measure this amount of power, you could most likley argue that the reading is not accurate.
Running a 10 mw led of such a meter would most likley not be recorded at all.

I've no practical experience of how electricity meters work, but I seem to remember when I was taught about power factors being told that these meters typically measure current, and that the mean square adjustment and integration over time was then used to calculate the transferred charge and hence energy. This was to explain why industrial consumers were concerned about power factor - because they are charged for current but get value from energy so they want the power factor to be as high as possible so they don't pay for energy they didn't use.

Given that these devices are specifically designed to measure current accurately over a wide range of conditions, I doubt that it will be easy to cheat them. You could demonstrate reducing the power factor so that the meter shows power that you didn't get, but there's no way (that I know of) to get a power factor greater than 1.

What scope do you have to interfere with the power supply coming in to the meter? Is it entirely under your control, are you allowed to use inductive coupling etc, or is it completely inaccessible to you? If you only need to demonstrate power going through the meter without being measured and the whole circuit is under your control, then you could try either using DC (I have no idea whether meters would detect that) of a very high frequency AC well outside the range the meter is designed to pick up.

PeterH:
....the whole circuit is under your control, then you could try either using DC (I have no idea whether meters would detect that) of a very high frequency AC well outside the range the meter is designed to pick up.

what do you mean by this?

Would inductive coupling be detected by the meter?

We have to power something, like a light bulb without the meter registering it.

That seems a very strange task, what exactly are you supposed to learn from it?

If you don't wire round the meter as suggested then the power will flow through the meter which will register it unless you tamper or mess with the meter in some way.

How about you try powering a light bulb with a series capacitor, that may throw off the phase angle enough to cheat the meter. Just be sure you use a non-polarized cap rated for well above the line voltage you are using. Look for 'motor starter capacitors'.

Lefty

David82:

PeterH:
....the whole circuit is under your control, then you could try either using DC (I have no idea whether meters would detect that) of a very high frequency AC well outside the range the meter is designed to pick up.

what do you mean by this?

Well, if the meter is designed and optimised to detect AC current then it may not detect DC current - or may not detect it well. If you have complete control over the circuit and can arrange for your supply to be DC rather than AC (or to have a significant DC component in addition to the AC) and IF the meter does not measure DC current then you'd be able to get power through the meter without it registering.

Alternatively, if the meter has been designed to selectively pick up mains frequency signals then it may not be able to pick if signals at a much higher frequency. For example, it's possible that if you send an RF signal down the line, the meter would ignore it (it would be filtered out). I'm speculating, because I have no idea what range of frequencies the meter deals with, but if you have full control over the circuit then it would be worth a try.

I don't feel optimistic that any of these approaches will work, but if you have a meter and time to play with it, what do you have to lose? Probably the most promising way to obtain power without it registering at the meter is to cheat. But that's hardly going to get you any credit when it's found out.

retrolefty:
How about you try powering a light bulb with a series capacitor, that may throw off the phase angle enough to cheat the meter. Just be sure you use a non-polarized cap rated for well above the line voltage you are using. Look for 'motor starter capacitors'.

http://www.galco.com/buy/NTE-Electronics/MSC250V161?gclid=COmO7sKh3LQCFWGnPAodOgwAqw

Lefty

yea, AC compressors use them right? So digital utility meters measure the phase angle to measure current?

Are you allowed to make a Tesla coil? You wirelessly power something if it was close enough. I'm not sure if you will be able to measure the current or not. If you think about it, the Tesla coil would be the power source and what your powering would be the load, and since they are not directly connected, you might not be able to measure any current.

It's just an idea.

PeterH:
....the whole circuit is under your control, then you could try either using DC (I have no idea whether meters would detect that) of
Well, if the meter is designed and optimised to detect AC current then it may not detect DC current - or may not detect it well. If you have complete control over the circuit and can arrange for your supply to be DC rather than AC (or to have a significant DC component in addition to the AC) and IF the meter does not measure DC current then you'd be able to get power through the meter without it registering.

Alternatively, if the meter has been designed to selectively pick up mains frequency signals then it may not be able to pick if signals at a much higher frequency. For example, it's possible that if you send an RF signal down the line, the meter would ignore it (it would be filtered out). I'm speculating, because I have no idea what range of frequencies the meter deals with, but if you have full control over the circuit then it would be worth a try.

I don't feel optimistic that any of these approaches will work, but if you have a meter and time to play with it, what do you have to lose? Probably the most promising way to obtain power without it registering at the meter is to cheat. But that's hardly going to get you any credit when it's found out.

We can't control the supply. It's just 240v. 60Hz. We can only create circuits after the meter. Does this rule out your ideas?

David82:
We can't control the supply. It's just 240v. 60Hz. We can only create circuits after the meter. Does this rule out your ideas?

Yes, it does. I don't think there is any practical way to get current through the meter without being registered by the meter, without tampering with the meter or upstream wiring.

OK, here's what you do. Get about a meter of EL wire. Get a lightbulb, dismantle it, and remove the filament and glass support. Shove the EL wire into the bulb and solder the end of it to the base such that you can plug it into a light socket.

When you screw the bulb in, it will light up. This will use....maybe 15ma... You'll have to wait a few days before the meter would record anything, .....if ever.

The color and intensity won't be up to what EL wire is capable of since you're only running it at 60hz, but it's a light that can be switched on and off.

Is it possible to draw a lot of current faster than the meter can detect? If so, how would you do that? By charging a bank of capacitors??

Of course it's possible, but how and how safe it is may be a problem. What kind of power meter is it? What is the supply input to it. This usually runs in big numbers: 50A, 100A, 200A?

If it's any of those numbers, you have a big problem outrunning it without blowing something up. Not only would you have to use extremely large wire (or run the risk of the magic smoke escaping) and some pretty sophisticated protective clothing. You're going to be channeling huge quantities of energy that can easily vaporize a few inches of copper, starting fires in insulation rated for 400C.

Sucking it faster than the meter can sense is not a good idea.

draythomp:
Sucking it faster than the meter can sense is not a good idea.

I've seen a video of that being done, and by the time you are putting in enough current to stop the meter from registering you need extremely heavy cables on either side and a supply good for tens of thousands of Amps - and the meter quickly collapses into a pile of molten slag. This is obviously highly dangerous as well as being contrary to the spirit of the assignment.

It's trivial to get a small bit of power without it being measured as mentioned above. Impossible to get a large amount because of the rules you have defined. So charge a Cap slowly, then power the light for 2 seconds.

Would inductive coupling be detected by the meter?

David82:
Would inductive coupling be detected by the meter?

Yes. The magnetic field you're taking in the secondary has to be replaced by the primary, using current. If inductive coupling didn't take any power then no transformer would take any power either.