When I got started in this business we used multi-meters with a d'Arsonval meter movement that was powered by the signal that was being measured. Of course the voltage that one read from the meter needle movement could be off a bit depending on the source impedance of the signal. They used to rate the meters input resistance in ohms per volt. Some would be 10,000 ohms/v or even higher sometimes.
If I had a multimeter rated at 1000 ohms per volt, set on the 10 volt range, what voltage would I read from the dial if I measured the voltage at the middle node of a voltage divider composed of two 10,000 ohm resistors in series, across a solid 5 volt DC supply? Later I bought a VTVM or vacuum tube volt meter, with a fixed input resistance of 11 megaohms.
Reply to Butch Shadwell
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The names of correct respondents may be mentioned in the solution column.
† Our friend Butch volunteers a bit too much, he never quite catches up to the current month.
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Measurements can be difficult. "using a multimeter rated at 1000 ohms per volt, set on the 10 volt range, what voltage would I read from the dial if I measured the voltage at the middle node of a voltage divider composed of two 10,000 ohm resistors in series, across a solid 5 volt DC supply? Later I bought a VTVM or vacuum tube volt meter, with a fixed input resistance of 11 megaohms."
Many students I meet, and some MS and PhD's as well, sometimes forget that whenever you measure something you have to take some energy from it. Measurement requires interaction with the phenomenon of interest. Before the advent of solid state electronics, portable meters had to function without the benefit of high input impedance amplification. The vacuum tube volt meters discussed last month required connection to the mains and so were considered a bench based measuring technology. There were several correct answers sent in this month. Adding the meter to the circuit changed the resistor divider from Ω to 1/3 of the 5 volt supply, and that is what would have been seen on the dial. With the VTVM the error would have been less than 1%, assuming the meter had that degree of accuracy in the display (which of course they did not). With these analog meters you rarely got more than two significant digits from the reading. Some of you may remember tapping the meter face trying to get it to settle at the right value.
But I bet you already knew that.
Butch
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