# Fuses ans stuff was : Lightbulb police?

Tony Duell ard at p850ug1.demon.co.uk
Thu Jun 10 12:44:34 CDT 2010

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> On Wed, Jun 9, 2010 at 15:50, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
> > One
> > classic was the time they told me that my method of measuing something
> > (actually capitance) was a 'comparison' and not a 'measurement'. To this
> > day I am trying to find a defintion of 'measurement' that does not
> > involve comparison to a standard.
>
> Tongue _firmly_ planted in cheek, I could ask you to measure the
> average number of peas in a pod! :-)

Sure. My standard of measurement is the 'unit' -- that is a set of one
object.

And I could claim that was 'counting' not measuring.

>
> My own feeling is that actually here you missed to learn something in
> school that is vital.  And that is an aspect of human interaction, or
> rather, interpreting unspoken demands.  Clearly, a successful student
> would know what the teacher wants, and solve the problem in that

I hate to say this, but my aim was to learn (real) physics. Not some
pretend version that the teacher wanted me to learn.

> fashion, rather than taking the literal meaning (and/or possibly more
> efficient solution) of the problem statement.  Since the subject is
> Physics, I guess the idea would be to measure capacitance using
> "lower" units of measurements: Capacitance can be found using time,
> current, and voltage - and volt can be derived (with difficulty) using
> a standard length, mass and time.  So, assume you have a calibrated

Actaully, you need one electrical base unit too. In the SI system, it's
the 'ampere' (which I have always thoguth was stupid, I regard the
coulomb as being more fundamental, being essentailly the charage on a
large number of electronics, not a rate of flow).

But anyway...

Every book I've looked in gives the bridge circuit (which is of course
what I described) as a method of measuring cpaacitance (and resistance,
and inducatance for that matter).  Fredrick Terman's [1] book
'Measurements in Radio Engineering' starts off with 'MKeasurement of
circuit constants at low frequencies' and describes all sorts of bridges.

Several other books point out that in general comparison to a standard of
the same time as the qunaitity being measured provide the most accurate
measurements.

[1] I assume every body knows who he was. No?. He was (IIRC) associate
professor of electrical engineering at Stanford in the 1930s. He had a
student who did an MSc project making a new type of audio oscillator
using a Wein bridge (RC) network an an amplifier whose gain was
controlled by a filamanet lamp in the feedback path. The name of that
student? William Hewlett [2].

[2] And what do you think the HP Mdoel 200 audio oscillator is?.

> ammeter, voltmeter, and clock, finding capacitance is easy!  Never

This actually brings up an interesting question to which I don't know the

Given the SI standards -- that is a copy of the prototype kilogram, a
current balance to determine the ampere, the appropriate atomic clock,
and whatever the length standard now is, how do you determine
capacitance?

Waht I do know is that the method this teacher was expecting is not the
answer, That method (based on a vibrating reed contact) has so many
systematic errors as to be totally useless.

> mind those calibrations were made by doing comparisons earlier.  Is is
> the best or most efficient way to do it? Hell no! But it's what
> physics teachers want.

As I said just now, I was there to learn physics. Not play games
second-guessing the teacher.

-tony

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