What do helmholtz coils do

It consists of two identical circular magnetic coils that are placed symmetrically, one on each side of the experimental area along a common axis, and separated by a distance h equal to the radius R of the coil. Each coil carries an equal electrical current flowing in the same direction.

A number of variations exist, including use of rectangular coils, and numbers of coils other than two. However, a two-coil Helmholtz pair is the standard model, with coils that are circular and in shape and flat on the sides.

In such a device, electric current is passed through the coil for the purpose of creating a very uniform magnetic field. Helmholtz coils are used for a variety of purposes.

This method is the simplest way to produce magnetic field for testing. It is very easy to vary the frequency and magnetic field under test. The second method is series-resonant method. This method is a powerful way to produce high magnetic field and very high frequency in the order hundreds kHz or even MHz. The third way is using a new current-amplified resonant method. This method generates the highest magnetic field density. The below sections will describe each method.

If the experiment is low frequency or the coils are low inductance or both, the Helmholtz coils may be driven directly using a waveform amplifier driver such as the TS Waveform Amplifier from Accel Instruments. Figure 6: Circuit representation of a Waveform Amplifier directly drives a pair of Helmholtz coils connected in series. Use the Equation-1 to calculate the coil current for a desired magnetic field.

Then use Equation-2 to calculate the maximum voltage is needed. Note the small parasitic resistance is ignored. The maximum voltage is when the current and frequency are both at maximum. The next step is to drive the Helmholtz coils with a high-current and high-frequency amplifier driver such as the TS function generator amplifier. At high frequency the coil impedance is very high such that high voltage is needed to drive high current through the coil.

For example, at kHz the impedance of a 2mH coil will be ohm. For most applications, this is not enough current to produce enough magnetic field. For high magnetic field applications, higher current through the coil is desired.

To drive a 2A high-current through the coil, V is needed! It is difficult to generate 5kV at kHz. To achieve high-current and high-frequency electromagnetic field, series resonant technique can be recommended. Figure 7: Waveform amplifier drives high current through the Helmholtz coils at resonance.

To operate the high frequency Helmholtz coils in resonance mode, a series capacitor is added as shown in Figure 7. The series capacitor impedance has an opposite polarity than the inductor. Thus the capacitor is acting as an impedance cancelation device.

It reduces the total impedance. At resonance the capacitor reactance imaginary portion of the impedance is completely cancels the inductor reactance. That is the inductor and capacitor reactance are equal magnitude but opposite polarity.

With only resistance remained, the function generator amplifier TS can drive high current through the Helmholtz coils LCR circuit even at high frequency. This method enables the signal amplifier to drive high current through the high frequency coils, but it can only operate at a very narrow frequency range near resonance. The disadvantage of resonant technique is that you need to change the capacitance when you change the frequency.

The series-resonant frequency of the Helmholtz coils is given in Equation The series capacitance, CS, is calculated using Equation The voltage across the series capacitor is given in Equation-2 above. At high frequency and high current, the voltage could be in the thousands volts. For example, at kHz and 1A current though a 2mH high frequency Helmholtz coils, the voltage across the capacitor is V! The capacitor must be rated for at least that voltage.

High-current Helmholtz electromagnetic coils discussed above can store enough energy to become an electrical shock hazard. Make sure all electrical connections are insulated with high-voltage insulators. Wires must be rated for voltages rating discussed above. Always disable the waveform amplifier output before connecting or disconnecting the coil and capacitor. Another resonant that is even more powerful than the series resonant is called the current-amplifier resonant.

This newly discovered resonant can boost the Helmholtz coils current by a factor of two. That is the coil current is twice the source amplifier driver current. Hence the resonant is magnifying current and the magnetic field. Helmholtz Coils A pair of conducting circular coils each having N turns, each carrying a current I , separated by a distance equivalent to the radius of the circular loops, produce a homogeneous magnetic field B in the mid-plane between the two circular coils. The strength of magnetic field is measured at a point in space often called the field point.

In the case of the Helmholtz coils, the field points of interest are located in the mid-plane between the two coils. As shown in the equation above, the strength of the magnetic field is dependent upon three quantities:. In the figures to the right, we can see how uniform the magnetic field is on the mid-plane when the spacing d is too close, just right, and too far. The figures to the right are calculated for coils with a radius of 20 cm i.

The following integral was evaluated to produce the three curves at the right.