A magnetic balance and an indicating coil for measuring a strong vertical magnetic field (the cyclotron magnetic field) are described. The magnetic balance consists of an aluminium rectangular coil, resting with agate knife‐edges on agate supports, and of a pair of pulley systems. It is sensitive to about 1 gauss in 10,000 gauss. The indicating coil has such a non‐linearity that, when it is working in the vertical position, it is able to indicate a field of more than 10,000 gauss and, on the top of this amount, a variation smaller than 1 in 10,000. Both of the apparatus have been tested, and actually used in conjunction with an alpha‐ray magnetic spectrograph.