What Is a Shunt Resistor?

Most people in the 1st year of any course that covers a wheatstone bridge are well aware that a voltmeter was an ammeter in series with a resistor.

Now, what could possibly be used to convert something with a low resistance to something with a high resistance...

Your point being?

The instrument needs to have the highest possible internal resistance other wise it loads the circuit, rendering its reading worthless. The only circumstance under which an ammeter would indicate a balanced stat e, would be when all four resistors in the bridge happened to have the same value. Not very useful.

Drivel. Having established a balanced state, you cannot know the value of t he resistor under test unless you know that of the variable resistor, so it needs to be calibrated. That is why such a bridge needs a calibrated switc hed resistor network, not some Heath-Robinson alternative.

Arty Effem wrote: [snip]

It always was, but you introduced both ammeters and voltmeters (erroneously). What is needed us a sensitive centre zero galvanometer, not an ammeter or voltmeter. The only purpose of the galvanometer is for indication *not* for measurement. The measurement is done entirely with resistors, the reference resistor and the ratio of resistances created by the use of the slide wire and knife edge. You can state all you like at the galvanometer and it won't tell you what the value of the unknown resistor is. It will just show "0".

In my day it was "first year at secondary school" stuff. However you seem to have been asleep through lectures and have never, apparently, attended any practical classes.

Here's a clue for you about how a slide wire Wheatstone bridge works. Quite frankly I'm astonished that you have never seen one of these, much less used one, as is clear from your comment.

Err, they both work on current flow.

They are generally the same thing - the ammeter using a shunt across it.

You seems to have missed the point that both are versions of an ammeter.

You don't seem to have a clue what they or a Wheatstone bridge is.

There is no current flowing in the ammeter/voltmeter/galvanometer in a wheatstone bridge when it's balanced, this does not require all 4 resistances to be equal.

You don't seem to understand how it works, you don't need a calibrated switched network, that's the point, you just need one calibrated resistor.

Thanks Steve,

I have 'O' level physics (1981), ONC and HNC (1988) in electrical engineering, and have never even seen a Wheatstone Bridge before, nor used one. I have heard of them, though.

David

David wrote: [snip]

I shall now retreat, muttering about falling standards of education. The Wheatstone bridge used to be on the JMB 'O' level syllabus and covered in detail for an HNC in electrical engineering.

I'm basing this on the fact that my father, HNC in EE, was able to help me understand my homework and give me some tips on how to do the practical classes.

It was certainly covered on the o level syllabus 1975 to 1977

You will have likely used one without knowing. It's the basis of capacitance and inductance measurement within a DVM, etc.

Weren't there different syllabi (?) in different areas ?

David

As in algorithms within 'chips, maybe ?

David

Sorry about that...

As an aside, a few years ago, one night I couldn't sleep. I got up and turned on the one-eyed monster and flicked through the channels. I found a programme on a BBC channel called something like 'bitesize' giving exam revision for students. This particular programme was stating that the Cathode of a circuit is now positive, and the Anode is the negative.

I thought I must of been dreaming.

David

It depends ...

Probably something to do with conventional current flow not being used these days - but calling the terminals anode and cathode makes a nonsense of that.

And in School Certificate 1945 to 1950. :-)

Don't remember a Wheatstone Bridge during ONC Elect 1950 to 1955.

The Wikipedia entry is surprisingly good at explaining the difference.