Step 2
A 3-dimensional magnetic field is generated by a 3D Helmholtz Coils System (3D-HCS), consisting of three pairs of coils oriented in the three spatial directions (see Fig. below [2]).
The 3D-HCS has the following specifications:
Coil |
Smallest |
Medium |
Biggest |
Diameter of the coil |
56 mm |
70 mm |
92 mm |
Distance between parallel coils |
28 mm |
41 mm |
46 mm |
Turns per coil |
349 |
362 |
370 |
Length of wire per coil |
70,2 m |
80,9 m |
117,4 m |
The intensities from each coil are controlled by the computer, which is connected to a DAQmx card from National Instruments (NI USB-6259) digital-analog converter in order to transform the simulated signal into a physical intensity. This device has the following characteristics:
- 32 analog inputs (16-bit); 1.25 MS/s single-channel (1 MS/s aggregate)
- 4 analog outputs (16-bit, 2.8 MS/s); 48 digital I/O (32 clocked); two 32-bit counters
Each pair of coils was connected to a different analog output channel so that the signal parameters of each can be controlled independently.
The calibration from intensity to magnetic field was performed for each coil by evaluating the average of maximum values for the generated magnetic field in the center of the system with a Hall sensor.
[2] The calibration curves are depicted in the figure below.

For applied intensities larger than approximately 35 mA, the system loses its linear behavior. The calibration has been calculated in the linear regime.
- X Coil: B(mT) = 0.29 mT/mA * I(mA) + 0.13 mT
- Y Coil: B(mT) = 0.30 mT/mA * I(mA) + 0.50 mT
- Z Coil: B(mT) = 0.13 mT/mA * I (mA) + 0.01 mT
The physical signals can be monitored during the operation with an oscilloscope. The system is equipped with a microscope and a camera for the visualization and the recording of the samples, respectively.
After long operation times, the system may warm up, especially for high intensities. This fact must be taken into account, since the increase in the temperature may affect the sample.