[NLRS] FRIDAY (Today) IS PI DAY

Fri Mar 14 18:27:27 EDT 2014

There was one circuit in the diagnostic panel of the Collins 821A-1 that 
I couldn't design with my slide rule. It took a couple days with a 
Friden calculator to iterate to a stable solution.

The general purpose meter was 100 microamps full scale and 1K 
resistance. I needed a meter circuit with two inputs and no meter 
circuit switching. One input needed to be 3 volts full scale and the 
other 10 volts full scale and each input had to load the metering 
dividers with 10K because the dividers were designed for that 10K load. 
1% resistors were available eventually.

So figure the multipler for 3 volts and 100 microamps, 29.00K. OK shunt 
that with 15K for the 10K input. Now the 10 volt multiplier needs 99K 
series and 11.11K shunt. But now that 111K is shunting the 1K meter 
changing the reading by making the meter circuit 991 ohms and taking 
additional current when driving the 3 volt input and leaving the 10 
multipliers attached. 3 volt multiplier and shunt has to change. Then 
the 10 volt multiplier has to change because of the change in the 3 volt 
resistors taking current from the meter, and the changes continue.
I could not do the parallels like I do on my HP calculator using sums of 
1/x repeatedly, had to take the product divided by the sum. Then when 
the iterations settled, look up the available 1% resistor values and see 
how much error there is, presuming the meter is perfect though it 
probably was only good for 3 to 5%. I couldn't stand the voltage drop of 
a diodes to switch and with the doorbell buttons on the panel for 
selecting circuits all SPST I had to make the two inputs work without 
switching. Oh, the meter was center zero so there's no way diodes would 
work.

I could only use the Friden when the department financial manager 
wasn't, I didn't have one for my desk and in 1963 there wasn't any 
electronic calculator available at any price. The first HP came out a 
few years later and the desktop was a few kilobucks.

That meter circuit worked fine for a few decades.

73, Jerry, K0CQ

On 3/14/2014 3:57 PM, Zack Widup wrote:
>
>
> Yes, that's true. I remember those days.
>
> And realize it was slide rules that got our space program going and put men
> on the moon.
> :-)
>
> 73, Zack W9SZ
>
>
> On Fri, Mar 14, 2014 at 4:36 PM, Dr. Gerald N. Johnson
> <geraldj at netins.net>wrote:
>
>>
>>
>> Back in the slide rule days 22/7 was close enough for most work. An error
>> of less than 1/2 part per thousand.
>>
>> 73, Jerry, K0CQ
>>
>>
>> On 3/14/2014 3:03 PM, Bill Ockert - ND0B wrote:
>>
>>>
>>>
>>> So the answer wasn't 42?
>>>
>>> -----Original Message----- From: W0WOI at aol.com
>>> Sent: Thursday, March 13, 2014 10:55 PM
>>> To: NLRS at mailman.qth.net
>>> Subject: [NLRS] FRIDAY (Today) IS PI DAY
>>>
>>>
>>>
>>>
>>> FRIDAY IS PI DAY:
>>> March 14th (3.14), is day. It's an occasion to _celebrate_
>>> (http://www.teachpi.org/) one of the most compelling and mysterious
>>> constants of Nature.
>>> _Pi_ (http://en.wikipedia.org/wiki/Pi) appears in equations describing
>>> the
>>> orbits of planets, the colors of auroras, the structure of DNA. The value
>>> of is woven into the fabric of life, the universe and ... _everything_
>>> (http://en.wikipedia.org/wiki/File:Life,_The_Universe_and_
>>> Everything_cover.jpg)
>>>
>>> .
>>> Humans have _struggled_
>>> (http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Pi_
>>> chronology.html)
>>> to calculate for thousands of years. Divide the
>>> circumference of a circle by its diameter; the ratio is . Sounds simple,
>>> but the devil is in the digits. While the value of is finite (a smidgen
>>> more
>>> than 3), the decimal number is infinitely long:
>>> 3.1415926535897932384626433832795
>>> 02884197169399375105820974944592307
>>> 81640628620899862803482534211706..._more_
>>> (http://www.geom.uiuc.edu/~huberty/math5337/groupe/digits.html)
>>>
>>>
>>> Supercomputers have succeeded in calculating more than 2700 billion digits
>>> and they're still crunching. The weirdest way to compute : throw_needles_
>>> (http://www.angelfire.com/wa/hurben/buff.html) at a table or _frozen hot
>>> dogs_ (http://www.wikihow.com/Calculate-Pi-by-Throwing-Frozen-Hot-Dogs)
>>> on
>>> the floor. Party time!
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