indicated by the analogy to a clock face similarly 16b 10 is a narrow 16b type longitudinal groove located at hour position 10 a second coil using wires in slots 16b 4 and 16b 8 provides magnetic coupling which reinforces that of the previously mentioned coil when a magnetic field is to be applied vertically as shown in fig2 therefore the upper and lower coils as mentioned above are energized to provide reinforcing magnetic fields at the center of the coil form 12 in the vicinity of cell 18 another pair of coils including a first coil utilizing grooves 16b 7 and 16b 11 together with peripheral groove interconnections and a second coil including wires extending down longitudinal grooves 16b 1 and 16b 5 serves to reinforce the magnetic coupling along the horizontal as shown in fig2 accordingly the structure as described hereinabove provides a relatively strong magnetic field along the axis of the coil form 12 and two relatively weak coupling arrangements to provide three mutually orthogonal magnetic fields or magnetic field coupling circuits fig6 is a detailed showing of one of the grooves 16a where it crosses a series of peripheral grooves 14 in this simplified showing of fig6 the circular nature of the.