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Thermal Effects on Resistance
This
graph shows the measured resistance of a solenoid under varying temperatures.
Any normal conductor will see a drop in resistance with a drop in temperature. With a small range of temperatures like shown here the effect is almost linear. The wiggles in the graph are due to inaccurate data generated by the measurement process.
To demonstrate this you need a solenoid of at least several hundred turns, an ohm meter or multi meter, and some freezer spray.
The resistance of the solenoid used for this test was 6.3 Ohms at room temperature. To increase the temperature of the coil it can simply be connected to a battery and allowed to heat up. the temperature was measured using an infrared thermometer.
You could cool the solenoid in a standard freezer to about -20, but it would take a while so we used some freezer spray to get quicker results. The lowest resistance from our coil was just 4.8 Ohms whereas the highest was 8.2 Ohms.
Reducing
the resistance of a coil means that a higher current can be drawn from
the same source of EMF (volts). This means that the magnetic field it
produces can be much stronger. When the temperature of a conductor drops
below a certain level its resistance will suddenly drop to near zero.
Under these conditions this is known as a superconductor. Superconducting
electromagnets are used in MRI (Magnetic Resonance Imaging) machines so
that ultra strong magnetic fields can be produced. These usually have
to be cooled with liquid nitrogen and require a lot of power.
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