ETS-Lindgren 6404 Helmholtz Coil - Manual (Page 19)

Operation | 19
Magnetic fields greater than one gauss may be generated at frequencies from
DC to 200 Hz. However, if higher frequencies are required, the coil impedance
will require fairly high driving voltages to drive enough current through the coil in
order to reach the desired field intensity. In certain cases, it may be necessary to
series resonate the coil if high driving current is required at a high frequency. If
this is the case, take precautions to avoid shock or burn from the high voltages
developed across the reactive elements.