# Tungsten resistance

1. Jan 14, 2014

### magmash

Hi guys

I have been playing around with a piece of tungsten wire by supplying it with 12Vdc and measuring the current draw to see how much the resistance changes when the wire is glowing hot. while doing this i am not getting any change in the resistance, the wire has the same resistance when its cool (no power supplied), and when its Glowing hot (supplied with power), shouldn't the resistance change when the temperature rise in the wire ? .

I am using Ohms law to calculate the resistance while the tungsten wire is being supplied with power
R=V/I, i am measuring the voltage with a Multimeter and the current with a Clamp meter (Fluke).

2. Jan 14, 2014

### M Quack

3. Jan 14, 2014

### magmash

I have had no problem with measuring DC currents, did the same experiment with a light bulb and got what i expected, higher resistance.

Does the resistance get higher with the temperature on every conductor Copper, Aluminum, ect ?

4. Jan 14, 2014

### sophiecentaur

To be more accurate, you can't be "Using Ohm's Law" in fact, because Ohm's Law requires the temperature to be constant. What you are doing is to use the Formula R = V/I, which is the definition of the Resistance. In this case, you will be measuring different values of resistance as the temperature varies so Ohm's Law is not part of what you are doing. Diodes and other devices are definitely not 'Ohmic resistances' yet you can still measure V/I and get a value for a resistance.

This is not 'the Maths Police' talking. I am just pointing our that confusion can arrive when a term like Ohm's Law is used in the wrong context. People can make assumptions and fall down a hole.

5. Jan 14, 2014

### Staff: Mentor

What are you supplying power with? A battery or a 12v power supply?

6. Jan 14, 2014

### sophiecentaur

This link has a graph of the variation of resistivity of tungsten with temperature. It covers a temperature range that you won't achieve but you can see the variation between room temperature (300K) and dull red heat (1000K). That's something you should easily achieve.
There may be something 'not optimal' (i.e. wrong) with your measuring arrangement. You need to be careful about where you measure the PD across the wire. For instance, do not include the connections to the power supply as there may be contact resistance where the current comes on and off the wire. Measure PD between two points near each end of the wire and also measure the current and volts simultaneously. If you are using the same multimeter for each measurement, the resistance of the meter and connections can be affecting the current and this can affect the voltage, with and without the meter.