Talk:Potentiometer (measuring instrument)

Pots as Measuring Devices

How are they used to measure anything? The only thing I can think is that they are used in conjunction with some other device -- I would bet that this other device is what is actually used to measure... thought? Jheiv (talk) 01:09, 26 June 2008 (UTC)

A potentiometer for measuring the potential (or voltage) in a circuit taps off a fraction of a known voltage from a resistive slide wire and compares it with the unknown voltage by means of a galvanometer. The slider tap of the potentiometer is adjusted and the galvanometer briefly connected to both the slider and the unknown potential. The deflection of the galvanometer is observed and the slider adjusted until the galvanometer no longer deflects. At that point the galvanometer draws no current from the unknown source, and the magnitude of voltage can be calculated from the position of the sliding contact. This null balance method is a fundamental technique of electrical metrology.

The potentiometer (pre digital) I used in an electronics lab was a drum about 40 cm long and about 16 cm diameter. There was a spiral notch around the drum and a 10 meter resistance wire in the notch which wrapped around the drum 20 times. Printed on the drum above the wire/notch were millimeter marks with centimeters numbered. I moved the slider until the galvanometer was about zero and then slowly rotated the drum by hand until the galvanometer was zero. If the reading on the drum was 500 cm, the voltage being tested at the slider was exactly half of the total voltage on the 10 meter wire. Before doing a measurement like this, you first had to calibrate the potentiometer using a mercury standard cell with the slider set at the cell voltage. Greensburger (talk) 01:55, 26 June 2008 (UTC)

Although it seems that it is, in fact, the Galvanometer, that is doing the indication, the integration of the potentiometer with the Galvanometer seems like a fair use of the term measuring device -- thanks for the explanation. Jheiv (talk) 04:10, 26 June 2008 (UTC)

The voltage is not measured by the galvanometer. The galvanometer only measures current flow to determine if two voltages are equal or not. The potentiometer measurement is a distance measurement on the spiral resistance wire which can have a precision of one part in 50,000 (one fifth of a millimeter on a 10 meter resistance wire). The unknown voltage is calculated from the distance measurement. Greensburger (talk) 16:40, 26 June 2008 (UTC)

The connection between the null-balance measuring method and Miller theorem

Miller theorem is a general concept that does not depend on the specific circuit implementation (tube, transistor, op-amp, etc.) It only requires two voltages in proportion V₂/V₁ = K to be applied to the two ends of an impedance element (a galvanometer here). In our case, the proportion is V₂/V₁ = 1 and it is obtained by a manually controlled voltage follower with series negative feedback. The supplying voltage source and the potentiometer constitute a regulated voltage source producing V₂ that is placed contrary and in series with the input voltage V₁. Thus the two voltages are subtracted according to KVL and their difference Vdiff = V₁ - V₂ is applied to the galvanometer acting as a zero indicator. The man monitors continuously Vdiff looking at the galvanometer and adjusts V₂ equal to V₁. So, this arrangement acts as a voltage follower and the resistance seen from the side of the input voltage source is virtually increased up to infinite. Circuit dreamer (talk, contribs, email) 17:57, 4 August 2010 (UTC)

Myself, I wouldn't put a link to such an opaque and irrelevant article. Could anyone who's not a black-belt circuit theorist read Miller theorem and explain how it applies to *this* article? It's about as relevant at Maxwell's equations, General Relativity or the Corn Laws. Links are supposed to help us understand things - I already know there's an indefinitely large body of baffling and irrelevant facts out there, I don't want them linked into every article. --Wtshymanski (talk) 19:50, 4 August 2010 (UTC)