A Weston standard cell – an introduction to voltnuttery?

Weston standard cells are pretty, but can they be more than a broken antique? And yes, that is liquid mercury visible at the bottom of the left leg.

One Cell

One Cell

I remember using Weston cells at school during physics lessons to measure voltages more accurately that could be done with a standard analogue meter (DMMs being years in the future). The measurements are based on using a 1m ruler plus resistance wire plus an uncalibrated NiFe cell as a bridge, and use a Weston cell to calibrate the bridge. That technique (which is a good example of using simple materials, understanding and imagination – rather than merely throwing money at the problem) is outlined below.

Anyway, I have vaguely wanted a Weston cell ever since, a little more so since I got hold of an HP3468 and have wanted to check whether it is still “accurate”. I’ve wondered about buying one on ebay, but they are unjustifiably expensive and sufficiently fragile that I doubt they would survive shipping.

Unfortunately I’ve now found a cheap one, and I fear it might lead me down the expensive path of voltnuttery. Memo to self: do not become a voltnut.

There’s a local school/college closing and flogging off its “stuff”, everything from desks and chairs, though bench PSUs 20MHz scopes and banana leads, to lumps of sodium and potassium. So naturally I pay a visit and have an extended rummage, since the very pleasant “salesperson” openly acknowledges she doesn’t know what’s what and can’t test anything, and anything she can’t sell she’ll have to pay to be taken away and disposed.

What do I come across on a desk but one “Cambridge Instruments Weston Normal Cell” and a dual “Muirhead Standard Cell” with a yellow anchor and MEL stencilled on the side. Clearly they are destined for the tip. Well, given their age they are unlikely to be “good”, and showing her the fragile glass means she couldn’t sell it on fleabay, so I offer her £5, and take them away.

Not expecting very much, I measure the voltage, and find the CI Normal Cell is 1.01865V and the Muirhead Standard Cells are both 1.01868V at 17C. OK; not broken, but it means I’ll have to find out a bit more about standard cells. Memo to self: resist becoming a voltnut.

On pulling the Muirhead apart and comparing them with an article in the “Muirhead Journal”, it appears that:

  • they are crystal locked saturated cells
  • 1.01859V @ 20C implies a 0.1N solution
  • serial number is almost illegible, but it 1702
  • date is almost illegible, but is 19-1-49 !

And I thinking that a 68 year old standard cell would be worse than useless!

I suppose I’ll have to see if I can find a 6/7/8 digit DMM, and learn about changes in the definition of the volt.    ….    Must. Not. Become. A. Voltnut.

How do you measure voltages accurately if all you have is one accurate voltage source, you must not draw any current from it, and the only available voltmeters would draw orders of magnitude too much current? That was the problem we had in school physics lessons; the techniques we had to use are a good example of the motto “There is a substitute for expensive equipment: imagination and understanding. But you can buy equipment”.

The technique uses:

  • a NiFe cell, which can supply reasonable currents but which has a poorly defined voltage (1.2-1.4V)
  • a Weston cell, which has a well defined voltage (1.0186V) provided no (< 1μA) current is drawn
  • analogue voltmeters which are 2% accurate and draw 50μA at FSD
  • a 1m ruler with a resistance wire along its length

Connect the resistance wire across the NiFe cell. The voltage on the resistance wire decreases linearly from one end to the other.

Use an analogue voltmeter to find the point on the resistance wire that is at 1.0186V (approximately, of course).

Attach the Weston standard cell in series with the voltmeter, and briefly touch the resistance wire at that same point. Ideally since both sides of the voltmeter are at the same voltage, no current should flow and the meter will indicate 0V. In practice it is necessary to move the contact point until the meter does not twitch.

Measure the distance of that contact point, X, from the end of the resistance wire; that can be done to 1mm, i.e. 0.1% which is much better than the analogue meter’s 2%. The NiFe cell’s voltage is 1.0186  \frac{1.0}{X} volts.

Voltages less than the NiFe cell’s EMF are determined by measuring the position of the null-point along the resistance wire. Higher voltages require a known resistive divider.


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