Author Topic: Measuring 2000+ Amps DC to the Hundredths Place in the 50's and 60's.  (Read 511 times)

Offline Patrick

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A question has arisen amongst some friends of mine as to how high current could have been measured during the 50's and 60's.

I have no doubt in our case that current sense resistors were enormously accurate and precise for that time and used for acquiring the initial signal. But the resolution is what has us perplexed. The difference between 2400.00 Amps DC and 2400.05 A DC is significant in our application and +/-200 mA is the difference between working and not working. Our modern instrumentation can handle this 0-3600 Amps easily but as a historical curiosity we were wondering how this was done in previous decades. The rate of tuning would be on the order of 10's of mA per second while we hunt for our desired value with a potentiometer. Typically we see 120-150 VDC on the main coil at 2200 to 3200 A DC.

A single analog meter movement could not be configured to span 0-3600 Amps and have the 0.05 A resolution with us nudging a needle up and down over the course of 30 seconds.

Is it possible Nixie Tubes or split-flap displays were used with early A-D schemes? Or for example, did they expect and configure the early current sense resistors of that era for 2000+400+25 = 2425 A full scale, and then put a 25.00 V meter movement over that last CS and be able to measure 2400 to 2425 A DC ? You don't have to measure the bulk current all at once, you pretty much know it accurately and precisely. But the final nudging up and down is whats critical and hard to resolve visually over such a large span, a precision meter movement sitting across the last (lowest value) current sense resistor solves that. On a different day they could break the circuit and put in a different string of CS resistors for other values, like 1740.50 Amps or 2922.3 Amps and then tune across the tiny 10.00 or 25.00 FS range accurately.

By using a traditional analog needle scheme the conversion rate (speed) and precision/accuracy wouldn't be an issue as it might be with the first generation A-D converters needed to drive a nixie or other type of digital display.

See current picture below:
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Offline davekni

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Not sure if this is relevant, but might be similar technique.  Worked for a chemistry professor summer 1975 on electrochemical cells.  Even though accurate digital meters were available by then, this professor insisted I use his ancient manual equipment.  Consisted of a precision 10V reference feeding a box of five decades of resistor divider switches and I think a potentiometer at the end.  Output of this decade divider box connected to one side of a galvanometer, other side to cell voltage.  Decade switches and potentiometer were adjusted to null galvanometer needle.
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Offline Patrick

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Thats interesting I hadn't thought of that.
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Offline klugesmith

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This doesn't directly address the titular question, but offers another example
that science often depends on extremely precise measurements.
In page 2 of this little story about the discovery of Hall effect:
https://web.physics.utah.edu/~lebohec/P5510/References/Discovery_of_Hall_effect.pdf
we see 24-year-old Edwin Hall in a university lab in 1879.  He configured a set of electrical instruments to explore resistance changes with sensitivity of 1 part per million.

Dave's story about matching unknown voltage with a decade resistance box reminds me of one early
digital voltmeter, whose goal was to improve the repeatability of readings made by factory workers.
It has a voltage reference, precision 10-turn potentiometer, and a voltage comparator. A servo motor runs a mechanical counter (as in odometers) up or down as required to make the display match the voltage being measured. I have a specimen somewhere, from early days of ebay.


Offline Patrick

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Ok so lets focus on the decade-null type solution.

How one implement this? If we have a 10mV and 3000 Amp current sense resistor how would we use a -1 to 0 to +1 analog meter movement and decade box with fine pot to drive the CS voltage to zero ?

I just feel like Dr. Lawrence and others would have found a clever, graceful and well thought out solution for this issue with what they had in the early 40's. Maybe even a twin  series current sense resistor string.

EDIT: I was going about this entirely wrong... They didn't have a single power supply powering a single coil on the main yoke. They had multiple small current sources all connected in series to a separate parallel winding. 5 coils of up to 500 amps for example and a 6th coil that added up to 50 amps. So parallel electrical circuits in series for a single magnetic circuit. They did the coarse additions with the large coils at 500 amps for 2.600 Teslas, then used the minor coil(s) at 22.3 amps to arrive at 2.604 Teslas. That makes more sense for the 30's and 40's. Really outsmarted myself on this one.

« Last Edit: July 22, 2024, 07:56:51 AM by Patrick »
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