Electric Meter Dissection

Unfortunately this is for an old electro mechanical meter. We have an electronic meter that sends the meter reading over the power wires to the power company and is no longer read at the house.

Very nice. Now, can you find someone to explain how the current affects the RPM of the disk? Is it regulated by friction, or something more reliable?
Bob Stuart

This shows that the meter is only measuring current at any given time. Power is current X voltage (1 amp X 1 volt = 1 watt). If the power companies are using nominal voltage such as 120 or 110 volts to multiply the current by in calculating watts then we could be paying for much more power than we are actually using since it is common for voltage to drop during high power usage. When the voltage drops on any given electrical device the amps increase to get the same power usage. We use the same power but get charged for more since they are only measuring amps.

The fact that I still have an electrical-mechanical type on my house makes me think that it must be costly to the electric company to remove an older type and replace it with a newer digital type. The cost of paying a meter-reader to go around from house to house must be a heck of allot cheaper. I guess they're "leaving well enough alone" until the old meter doesn't work anymore.

so, shouldn't the system be measuring by ohms at the outbound to have a fair and accurate calculation of what has been used?

Bill.Redding,
Power is delivered to a single phase meter at 240 vac, not 120. Single phase services have two hot conductors that are derived from two separate transformer taps, allowing both 120vac and 240vac to be available at the load side of the meter base. Both phases are monitored and measured.

I think everyone is missing the point. All the times I had a mechanical meter at my house, and to lower my electric bill all I had to do was a little gear switch night ops. My electical knowledge is at a novice level so don't beat me up to bad if I'm wrong, but with another resistor added to the connection between the house and the motor, I could probably take the voltage down thus lowering amps, and causing the dial to spin slower. Hmmmm. Okay I'm kidding. I'd probably break something anyways.

doitbetternow,

I understand residential service is provided a neutral or ground wire and two 120 VAC wires that are 180 degrees out of phase so that the voltage between the two hot leads gives 240 volts. This does not change the fact that if the mechanical meter only measures amps and multiplies this with a fixed gear to give power consumption (watts) then the voltage at the time of use is not taken into consideration. Power grids typically have voltage drops during high power usage such as hot afternoons when air conditioners are used. If I am being supplied by 110 VAC instead of 120 VAC then my air conditioner will draw more amps to produce the same results and I will be charged more $ for the same power consumption than when I am supplied with 120 VAC.

Hi All - Yeah, I realize that most homes no longer have this style of meter. The intent of the article was really just that it's neat that the mechanical device is performing finite integration. The wheel and disk integrator actually has an interesting history including Vannevar Bush's differential analyser, which is worth reading about if you've not already.

@bill.redding - I may not have made it totally clear in the accompanying article, but the speed of the wheel (and thus the tally of your power usage) is controlled by both the voltage and the current. The large conductors shown in pictures 6 and 7 wrapped around the magnet are responsible for the part of the magnetic field proportional to the current. Above those heavy conductors in picture 6, you'll also see a black box with two blue wires coming out of it. That is another set of windings around another ferrous core which is creating a magnetic field proportional to the voltage. The net resulting magnetic field is the vector sum of the voltage and current magnetic fields.

@BobStuart An eddy current resulting from the conductive but not magnetic aluminum disk sitting in the magnetic field discussed in the previous paragraph is what causes the rotation of the wheel. Now, why the resulting speed of the aluminum disk is proportional to V * I rather than V + I (given that the magnetic fields representing the magnitudes of V and I are added, not multiplied) is something someone with a better understanding of magnetic fields and eddy currents than I is going to have to explain.

What else would you like to see dissected?

- Vin Marshall

Great, great, great article. I just want you all to imagine engineering this device and not about getting the shaft from the power company, because this device does in fact take into account voltage and current. It's not 'pre-set' to 120 Vrms. The eddy currents themselves produce electromagnetic forces, which is essentially why the aluminum disk does in fact turn. The eddy currents are proportional to the magnetic field being produced by both the current and voltage coils... all in all, simple concept, but the mathematics(integrator), and gearing of these machines are neat. And just because they are not digital does not mean you need a new meter, but I'd say that depends on how old your meter actually is. Who knows, you may be using more power than you're being charged for if your meter is old... I will look into the effects of old coils and magnetic field-production and get back to you, but I'd say be hesitant before changing out that meter.