Excitation Current The simplest way to measure the resistance of a sample is by passing a DC current through the sample and using the 4-wire method described above. There are two problems with this, however. First, there is the problem of thermo-electric potentials between two metals. For example, if a connection is made between high resistance sample leads and ordinary copper wire, there will be a temperature dependent voltage across the junction. The simple solution to this is to do two separate measurements, the second one with the current direction reversed. Taking the average of these two readings would cancel the thermo-electric potential. One potential complication of this method is that the thermo-electric potential changes with temperature. If there happens to be a temperature drift during the time interval between the two measurements, the thermo-electric potential will change, and then the average of the two measurements will not give an accurate result. However, this is usually not a problem. Another problem associated with pulsed magnetic fields is that there is a huge voltage pick-up from the pulse of the magnet at about 40 Hz. The cause of the voltage pick-up is the change of magnetic field with time, dB/dt. When the flux through a loop of wire changes, a voltage is induced in that wire. The voltage is directly proportional to dB/dt, and to the area enclosed by the loop of wire. This problem is mitigated by using an AC excitation current and a lock-in amplifier. The lock-in amplifier has an internal oscillator that acts as the current source. It then reads the AC voltage across the sample. With a high enough frequency, the dB/dt signal gets rejected by the narrow bandwidth of the lock-in amplifier. Typically the frequencies are between 30 kHz and 100 kHz, but sometimes as high as 500 kHz, or as low as 10 kHz.