Retrotechtacular: Weston Electrical Instruments

A ‘meter is one of the most important tools on any electronics bench. After you’ve exhausted your five senses trying to figure out what’s happening in a circuit, firing up the old ‘meter is usually the next step. Meters are largely digital nowadays, but their analog ancestors are still widely available. We have a chemist and inventor named [Edward Weston] to thank for the portability and ubiquity of DC measuring equipment.

After immigrating to the United States from England with the degree in medicine his parents wanted him to earn, [Edward Weston] asserted that he was more interested in chemistry. His career began in electroplating, where he soon realized that he needed a reliable, constant current source to do quality plating. This intense interest in power generation led him to develop a saturated cadmium cell, which is known as the Weston cell. Its chemistry produces a voltage stable enough to be used for meter calibration. The Weston cell is also good for making EMF determinations.

Within a few years, he co-founded the Weston Electrical Instrument Corporation. The company produced several types of meters along with transformers and transducers known for their portability and accuracy. In 1920, [Weston & Co.] created this 1920 educational film in cooperation with the United States Navy as part of a series on the principles of electricity.

The viewer is invited to consider the importance of measurement to civilization, most notably those fundamental measurements of length, mass, and time. [Weston] positions his electrical measuring instruments at this level, touting them as the international favorite. We get the full tour of a Weston meter, from the magnet treated for permanence to the specially designed pole pieces that correctly distribute lines of magnetic force. What education film about electromagnetism would be complete without an iron filings demonstration? This one definitely delivers.

Retrotechtacular is a weekly column featuring hacks, technology, and kitsch from ages of yore. Help keep it fresh by sending in your ideas for future installments.

21 thoughts on “Retrotechtacular: Weston Electrical Instruments

  1. CSB time-

    a friend of mine has a weston (style) cell and one of his hobbies is metrology.

    first time I was at his place, I saw that old looking cell and saw a label attached saying ‘do not measure!’. huh? that was a strange label to find on a voltage standard.

    well, I then learned that even a very very high z meter would be too much of a load on this and it would ruin the cell, by taking even a tiny bit of energy from it.

    the proper way to measure is to put in an equal and opposite voltage in series and watch your ‘null meter’. and keep it quick!

    but that label on the battery just struck me as funny and I never forgot it.

  2. I used a Weston cell at college for something. I can’t remember what we were measuring, but the cell stands out in my mind.

    …so when I saw one at an antiques shop, I had to get it.

    As old as it is, it’s still more accurate than my voltmeter by a full digit. I used it to check the calibration of my Fluke DVM versus the Radio Shack DVM versus the free ones you can get from Harbor Freight. Curiously, all DVMs I’ve measured to date are fairly accurate.

    http://conradhoffman.com/stdcell.htm

      1. if you directly measured it in the ‘usual way’ then your cell is now ruined, technically. it may have already been pre-ruined (lol) but at any rate, you never directly attach dmm or scope leads to ref cells like this.

        very likely, any weston cell you buy on ebay will also be ruined unless its been kept by a chain of owners who all knew the handling rules for such things.

        1. Actually you can draw current from it, but no more than 1uA. And more than 100uA will ruin it as voltage standard. Sp one might use a high impedance buffer with it to get his 1.0185-1.0187V standard. Also one can regenerate this cell by replacing chemicals with fresh ones. Be careful though, mercury and cadmium are quite dangerous.

        2. As mentioned in another post, I can draw 1uA from the cell.

          My DMM has an input impedance of 10 megohms (yes, I measured it), and the Weston cell voltage is 1 volt. Measuring directly with a DMM will draw 0.1 uA, and will be perfectly fine.

          The warning about direct measurement was for analog voltmeters using an actual meter. Those devices drew considerable current to move the meter needle.

          Modern DMMs will be fine and won’t ruin the cell. Any current drain will affect the cell, but at the 0.1 uA level it takes many hours to have an appreciable change.

          My Fluke DMM reads 1.013, the Radio Shack reads 1.019, and the cell nominal voltage is 1.019. The cell is very old and the voltage decays with age, so I tend to trust the Fluke more than the RS device.

          I don’t now if it was ruined or not by previous owners, but it seems OK and it’s an interesting conversation piece to have around the lab.

  3. Decades ago I built a Heathkit VTVM (Vacuum Tube Volt Meter) from a kit. The calibration instruction said to use the 1.5v scale to measure a brand new carbon D cell. There is a little dot just beyond the 1.5v mark which is the voltage of a brand new cell. You adjusted the meter movement to put the needle on the mark.

    Any brand new primary cell will produce a voltage for calibration that is good enough for most hackers purposes. The challenge is to know the exact chemistry in the cell, the voltage it produces, and making sure wasn’t sitting on the shelf self-discharging for months.

    Still have the meter.

  4. for not too much money, you can buy calibrated refs from hobbiests. dmmcheck is one box (I have one) and it gives measured values on stable (or, affordable-stable, lol) standards. the meter that guys uses has ‘2 more zeroes’ than I do, so I’m all set, as are most who take the ‘affordable’ way out.

    almost all of the test/meas gear in my home lab is cheap used ebay sourced gear. but I picked the better models and brands and looked for things that were not abused. all were old and were hp,tek,fluke,keithley and those kinds of brands. what I found was that every unit was still in cal (by checking with my dmmcheck), and nothing needed changing other than cosmetic cleaning. the flukes and hp’s and (etc) all kept their calibration stable after years, sometimes decades. not one needed to be changed.

    and so, if you don’t abuse your gear and you buy proper brands and models, you may not HAVE to recal over short term, and maybe not even over long term.

    1. That is definately true for home/hobby uses. But as soon as you are in a corporate environment, you should follow some quality insurance guidelines, and having a recent (usually no older than 24 months) calibration certificate of your equippment from an accredited lab is basically a must. The paperwork and documentation we have to go thru at work, just to uphold the quality-management requirements and pass all the audits can get really annoying and expensive.

    2. Yes I understand this topic is over 3 years old, but just now ran across it while doing some back ground on an article in the September issue of nuts volts, of a voltage standard project. When shopping for a DMM, for kicks I checked out the offerings from Fluke. Turns out to have your purchase calibrated to a standard, would incur a substantial extra fee. Not that I’m saying the run of the mill straight out the door Fluke product, would be a problem, just pointing out, as always everything is relative.

  5. Weston Instuments, Inc., was acquired by Schlumberger ( a huge French holding company) in the 1960’s.
    Before the acquisition Weston Instruments was a leader in electronic measurements and innovation.
    Weston developed and held the patent on “dual slope integration” and other analog and digital innovations.
    Schlumberger ignored Weston and allowed it to gradually sink into obscurity and demise, a loss for America.

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