Objective:
– To show how resistance of an electrical conductor will increase with an increase of temperature.
Apparatus:
- Resistance demonstrator
- Power supply (Circuit-Test PSB-330T), or other power supplies with numerical readouts
- Propane gas tank, with valve and hose
- Large Analog Ammeter, 0-2 Amp DC (Triplett 726)
Method:
The resistance wire coil (mounted above a burner) is connected in series with the Ammeter and Power supply. While the coil is cold, current is adjusted to *1.7 Amps. See preferred settings below. When the burner is ignited, current flow will rapidly drop to approximately 40% of full scale, due to increased resistance of the wire coil at a higher temperature.
Note:
Do not attempt to use a lamp as an indicator of current. (A lamp filament behaves like a negative resistance as voltage decreases, masking the desired effect).
Settings:
On the Circuit Test Power Supply, turn the current to minimum, and adjust the master voltage knob until voltage reading turns on. Since we want the circuit to vary in current, we want the CV light (greenish yellow light) to turn on to have the circuit in constant voltage (CV) mode. Recommend settings when the power supply is in CV mode: V= 1.3v, A=1.28 amp, and on the large analog ammeter, A=~300 mA. You can try to adjust the voltage knob to a different value as long as the current is under the 500mA the max current that the analog ammeter can read.
* The 1.7amp can be varied, as long as it is in CV mode and it is within the limits of the analog ammeter.