[SeanB]

Digital meters use integration, selecting the time constants of the integration so that it is a multiple of both 50 and 60Hz mains, so that the integration is done over complete cycles, leaving a stable reading. Typically, for the millions of clones of the venerable ICL7106/7 chipset, this equates to 2.5 readings per second, making a 3.5 digit meter something that can be gotten for literally cents, and with linearity only determined by how accurate the reference voltage you use is at being stable, and the 2 film capacitors used to do the integration and zero offset adjustment, plus how stable short term your clock is. High frequency response is dictated more by the method you use to compensate for stray capacitance, though the more expensive meters also use dedicated true RMS converter chips, which can give a true reading irrespective of the waveform, including working on DC as well, though they do have a limited frequency response, but can do a very good job, and as well some are set for true RMS, but others in the same family of meters, using the exact same chipset, are, due to them being used historically in the military, are average responding, because it is near impossible to rewrite all the old service manuals that were done when analogue moving coil meters, average responding, were the only thing, and a digital voltmeter was something both frightfully expensive, esoteric and not mobile easily. But digital is now cheaper than analogue, especially if you want accuracy and repeatability, so the coin is now flipped.

[Bored chemist]

Digital meters use integration, selecting the time constants of the integration so that it is a multiple of both 50 and 60Hz mains,

Other frequencies are available.

You might also try one of these- a pretty good RMS measure independent of crest factor and frequency over a very large range.
https://en.wikipedia.org/wiki/Ammeter#Hot-wire

[SeanB]

Other frequencies are available.

You might also try one of these- a pretty good RMS measure independent of crest factor and frequency over a very large range.
https://en.wikipedia.org/wiki/Ammeter#Hot-wire

Yes, but not at all common these days, mostly reserved for RF measurements. Fluke and Agilent offered the meters, but both companies no longer can supply the delicate balanced thermal measuring unit that is at the heart of them any more. They lost the technical ability to make them.

The nice thing about choosing to be compatible with 50/60Hz as integration frequency is that you also get good repeatability with the other common supply frequency of 400Hz, and almost all meters that are calibrated for 50 or 60Hz will have similar accuracy at 400Hz, though you can also get them certified for this, along with up to 20kHz if desired. Yes most common DMM chipsets will work up to 100kHz no problem, though some will exceed that by a large margin, often up to 100MHz on some, with reduced accuracy. Really depends on how it is laid out and what care you take with using the reference design.

[paul cotter]

And don't forget the thermocouple rf ammeter. These would be rms tracking, I would imagine. I have one somewhere, buried under mountains of junk.

[JesWade21]

DC can be transmitted over long distances if the voltage remains constant. Because increasing voltage in the AC world has historically been far easier and less expensive, we use it. DC connections can be found everywhere, including every major interconnect.

[evan_au (OP)]

DC connections can be found everywhere, including every major interconnect.

This applies specifically at interconnects between:
- Different frequency networks: One side of Japan is 50Hz, the other is 60Hz. So when the east-coast earthquake knocked out power stations, they had limited capacity to buy electricity from the west coast through DC interconnects.
- Different phase networks: Texas runs at nominally 60Hz (same as the rest of the USA), but is not phase-locked to the rest of the USA. So when a cold snap froze gas plants in Texas, there was limited ability to buy electricity from the rest of the USA through DC interconnects.
- In phase-locked grids, the cheapest interconnects are AC, powered by 3-phase transformers
- But, driven by the revolution in high band-gap semiconductors, the price of AC-DC and DC-AC conversion is dropping rapidly. This provides other advantages, like long-distance transmission lines with no power factor, and software-controlled power-factor compensation.

[JesWade21]

In Europe, the standard mains voltage is 230V plus 10%. In reality, however, the UK continues to use 240V and the majority of the rest of Europe continues to use 220V, just as they did prior to the introduction of that universal standard.

[Bored chemist]

"Why is mains electricity 240v"
Because it's a dozen score.

[paul cotter]

No, you are all in error: that voltage figure was decided by the pink unicorn. End of story.