16 Apr
2011
16 Apr
'11
11:06 a.m.
On 2011 Apr 15, at 11:47 AM, Tony Duell wrote:
I expect that what your instructor was getting at was that the use of
Actualyl, I know exactly what the teacher wanted. He wanted me to regurgitate
the method in the standard text book. A method involving a vriating reed
relay, (frequency known) charging the capacitor from a DC supply and
discharing it through a (milli)ammeter. This method (whcih si only found
in school physics books :-)) has a numebr of systematic errors, not
lreast it assumes the charging and discharign time constants are much
less than the period of the relay vibration. This is not always the
case... And I was not there to repeat the answers fro mthe textbook. I
was there to learn physics, which means thinkign about the question.
But anyway... if you look in any serious book on electrical measurements
[1], you will find the bridge circuit desribed as a way of _measuring_
'circuit constants'.
[1] For example 'Measurements in Radio Engineering' by Fredrick Terman [2]
[2] You do know who his is, right?
On 13/04/11 14:45, Dave McGuire wrote:
> If someone tried to feed me that garbage in a school, I'd demand my
> money back. Bravo for trimming the fat from your stuff.
Actually, that is minor compared to some of the garbage I was 'taught'
at
school. I fact I don;t think I ever had a clueful matehmatics of
physics
teacher.
I think the wordst was when I was aksed to descrtivbe a method of
measuring something (the capacitance of a capacitor). I did so (an AC
bridge) and was told 'That is a compariston and not a measurement', I
am
still waiting for a definition of 'measurement' (or an esample of one)
which
does not involve a comaprison to a standard. an AC bridge typically involves comparison with
another C, which also
Actually, it's possible to balance a R-C arm agains an L-R arm. Which
means that you could use a stnadard inductor. Practiclaly, this is used
ot measure inductance (against a known capacitor) becasue standard
inductors are non-tirival to make reliably.
needs to be measured. It becomes a 'turtles all
the way down' problem.
Sure. However, if I ask 'how would you measure the width of a sheet of
paper to an accuract of around 1%', I will bet everyone will say 'with a
ruler'' That is comparing one length against another.
This would be in contrast to a solution which examines
Cx in terms of
the definition of capacitance (time/charge) and breaks it down to more
fundamental/axiomatic measurement units (i.e.,time,mass), even though
this does involve comparison to some other standard.
Sure. If the teacher had asked for a method of 'measuring capacitance
against the fundamental SI standards' then that's a different question
with a ddifferent answer. Had he commented on by bridge method and said
something like 'OK, now do it without needing a known capacitor' that
would be rasonable too. Btu to say 'That's not a measurement, it's a
comparison' shoews, IMHO, a lack of understanding as to what a
measurement actually is.
"That is a comparison and not a measurement"
may not be a full
expression of the distinction, but I think your instructor had a point.
I am still not convinced....
I'm not up to scratch on my measurement theory, what are the
fundamental standards these days?, there's oscillation of the cesium
atom for T, there was the Meter bar in Paris for D, but hasn't that
When I was at school. 'lenght' was defiend in terms of the wavelngth of
some spectral line (I forget which one). I think that now, c (seped of
light) has a defined value, and that lenght is defined from the distance
light travels in a certian time.
Mass is still (AFAIK) based on a standard prototpye (it's the only unit
that is)
The electrical unit that was picked is the ampere, defined in terms of the
force betwene '2 infintiely long, infinitley thin wires'. Of course
practiually, toyou use coils and correct geometrically. I always thought
the colomb (being essentially a number of electrons) was a more
fundamental unit but anyway. And the volt is easier to determine
accurately, using a microwave-excited josephson junction (I did this
experiment at university, it's tricky to get it to work, but if you get
the 'votlage staircase' then the steps are the right size).
I think the mole is defined from the number of attons in a certain mass
of a certain isotope of an certain element (carbon 12 IIRC)
The kelvin is defind in terms of the triple point of water (or it was
when I learnt such things).
My Rubber Book is ytears out of date, so some of those may have changed
-tony