Il giorno Fri, 30 Mar 2018 09:32:13 +0200 Edward Bartolo <edb...@gmail.com> ha scritto:
> On 29/03/2018, Simon Hobson <li...@thehobsons.co.uk> wrote: > > Edward Bartolo <edb...@gmail.com> wrote: > > > >> What you wrote reminded me of a dangerous filter that consists of two > >> high voltage series-connected capacitors connected in parallel with > >> the mains with their middle point earthed. Since these capacitors are > >> almost certainly the same value they will devide the mains voltage by > >> two. This fact can be verified with a high impedance voltmeter > >> connected with the disconnected earth connector and one terminal of > >> the mains supply. > > > > Yes. Doesn't need to be that high an impedance meter either, a while ago > > (as part of a discussion in another forum) I just did a quick check with > > a basic digital multimeter and measured about 95V on my Macbook Pro. And > > as I mentioned, there's enough current there to be able to feel it. > > When I was studying Physics for my A Levels and at university, all my > professors insisted upon using high impedance voltmeters. A cheap > digital voltmeter/multimeter can easily have an impedance of a few > megaohms which is quite high. > > I remember, once I had a practical Physics experiment that involved > getting the characteristics for a transistor. My fellow students used > old moving coil voltmeters and got terrible results. I asked to be > granted permission to use a digital voltmeter for the experiment and > got excellent results for all three characteristics. After, that the > university physics lab was equipped with new digital voltmeters. When I was doing the same kind of studies and lab sessions we were purposely given analog voltmeters and our task was to: 1) rate the voltmeter's internal impedance; 2) measure resistors' impedance; 3) apply the necessary corrections to the mearurements in (2) to get much better than "terrible" results. The point was that on students more emphasis was put in learning the right procedures and to discern and correct the various factors that affect lab equipment correctness rather than getting the best results in the shortest and easiest possible way, which is what professionals are expected to do when they carry out their duties. Alessandro _______________________________________________ Dng mailing list Dng@lists.dyne.org https://mailinglists.dyne.org/cgi-bin/mailman/listinfo/dng
