# [time-nuts] OT: Basics of voltage calibration?

Bill Hawkins bill at iaxs.net
Sun Mar 15 02:39:14 UTC 2009

```John,

Try for older texts on electricity, 50 years or more back, before things
got complicated.

The trick with potentiometers is to establish a precision current in a
set of resistance decades, so that E = I * R gives a precise voltage. A
galvanometer (or microvoltmeter) is used so the unknown doesn't change
the precision current.

A resistor and a millivolt trim resistor establish the standard cell
voltage using the same precision current in the divider string. The pot
that establishes the standard current from a working battery is adjusted
until the voltage across the standard cell resistor nulls with the
voltage from a primary standard cell.

Fluke differential voltmeters use these principles, although they don't
use standard cells. You can easily calibrate a Fluke DVM (not digital)
to a standard cell.

The thing is, standard cells don't like to be disturbed. Shipping would
change the equilibrium of the electrolyte and invalidate the millivolt
calibration. You can't ship primary standard cells. But they were used
in portable devices like thermocouple potentiometers, because 1%

I have an extra Fluke DVM around here someplace, if you're interested.
Free to a good home for experiments.

Bill Hawkins

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of John Ackermann N8UR
Sent: Saturday, March 14, 2009 8:34 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] OT: Basics of voltage calibration?

I'm interested in learning some basics about precision voltage
calibration (as can be realized by the hobbyist, not Josephson Junction
systems!).  A Google search hasn't turned up anything like a tutorial.

Anyone know of any good app notes or other references on things like
standard cells, zener references, precision potentiometers, etc? -- and
how to use them?

Thanks,

John

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