reference article: D. Andreuccetti, M. Bini, A. Ignesti, R. Olmi, S. Priori and R. Vanni, Helmholtz coils for the generation of standard magnetic fields in biological applications. Proceedings of the VII National Congress of the AIFB, Monte Conero, Ancona (Italy), June 1992. Phys. Med. IX (Suppl. 1), 283-285 (1993).
The HC-50 exposure system consisted of a Helmholtz pair system comprising two parallel, coaxial, circular coils having a diameter of 40.6 cm. The distance between coils was equal to their radius (20.3 cm). Each coil was made up of 16 turns of 0.5-mm-thick enamelled copper wire. The two coils were connected in series so that the same current flowed in both. The field strength was regulated by varying the drive current supplied by an adjustable power unit allowing the HC-50 to produce a 50 Hz magnetic flux density varying from 1 to more than 150 µT.
Each lymphocyte population was divided into two aliquots. The exposed cell aliquots were incubated at the desired field intensity in a cell incubator containing the HC-50 Helmholtz coil system, and the unexposed cell aliquots were placed in a second incubator used in parallel. As an additional control, some experiments were performed by reversing the current in one of the coils in the HC-50 (sham exposure).
The actual magnetic flux density is a vector function of the position inside the Helmholtz coil system (being exactly equal to the nominal magnetic flux density in the system center only), but calculations and measurements show that intensity and direction errors with respect to the nominal magnetic flux density are less than 1% in a spherical volume centered in the system center and having a radius equal to one third of the coil radius, i.e. approx, 7 cm in the HC-50. The HC-50 power supply unit was equipped with a solid-state current meter whose LCD was calibrated to indicate the nominal magnetic flux density in microtesla. Scrupulous construction and precise calibration of the current meter guaranteed the overall accuracy, which was within 3% of the 7-cm-radius spherical volume. Although the system accuracy is too high to be assessed with standard magnetic flux density meters (rather the HC-50 itself can be used as a calibration device for such probes), it was checked and confirmed as far as possible with a professional commercial magnetic flux density meter (EMDEX II by Enertech consultants); the same instrument was also used to evaluate the background field intensity in the incubators and in the environment where cell exposure took place.