# The hot wire ammeter

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1. Jan 22, 2017

### Samar A

In my text book it says:"in the hot-wire ammeter

the pointer stands at a definite reading when the temperature of platinum-iridium becomes constant and the wire stops expanding. That is achieved when the rate of heat radiated by the platinum-iridium wire becomes equal to the rate of heat generated in it."
I actually don't understand the phrase: "the rate of heat radiated by the platinum-iridium wire becomes equal to the rate of heat generated in it" could you please clarify it for me?

2. Jan 22, 2017

### Staff: Mentor

It is really the same thing as saying that the temperature stops changing.

$I^2 R$ losses generate heat in the wire. The hot wire looses heat to the air around it. When the two are not balanced, temperature changes.

3. Jan 22, 2017

### Averagesupernova

Interesting. I had not seen the hot wire ammeter mentioned anywhere for many many years. I always thought it was a clever setup in an as-simple-as-possible way.

4. Jan 22, 2017

### Samar A

We study AC circuits in our physics assignment this year, and we studied how the moving coil ammeter works to measure the intensity of the DC, but it couldn't measure it for an AC, so we should know about the hot-wire ammeter.
I noticed the same too while searching on the internet. The hot wire ammeter is not mentioned so much, and there aren't a lot of explanations about how it works.

Last edited: Jan 22, 2017
5. Jan 22, 2017

### cnh1995

The amount of heat generated in the wire in a particular time is proportional to the square the rms current through it. I think this is the principle used while calibrating hot wire meters. RMS value is also called as 'heating' value.

You will study moving iron type meters where the driving torque is proportional to the square of the current. They are preferred over hot wire meters now-a-days for measurement of ac quantities

6. Jan 22, 2017

### Samar A

Yes, exactly, that is the principle used. But what confused me is that there is a heat generated and a heat radiated. I don't know if there will be an equilibrium so that the wire stops expanding or what it meant by saying that the rate of heat generated becomes equal to the rate of heat radiated.