Three methods have been devised for the measurement of the radio-frequency voltage which is applied to the accelerating electrodes ("dees") of a cyclotron.
The first method was one suggested by Professor W. F. G. Swann. The principle involves measurement of the magnetic field in the immediate vicinity of one of the conductors leading to the dees, and of obtaining thereby the current in the conductor. The magnetic field is determined by utilization of a small coil placed with its plane perpendicular to the lines of magnetic force and with its center at the point where the magnetic field is desired. The coil forms the inductance element of a simple circuit, whose other element is a capacity in series with it. The magnetic field is determinable immediately from the current in the coil circuit at resonance and from the resistance corresponding to the frequency. The resistance may be obtained without any further assumption by the usual method of measuring the current at resonance and also with a value of capacity differing slightly from that corresponding to resonance. The resistance may be obtained also by making two measurements of the current at resonance, the first being made with the normal high frequency resistance of a circuit, and the second, after the addition of a known resistance of a type unaffected by frequency. Still another method involves using permanently in the circuit a known high frequency resistance R sufficiently great to dominate the * This study was made on the cyclotron of the Biochemical Research Foundation of the Franklin Institute built by Dr. A. J. Alien and Dr. M. B. Sampson and located at the Bartol Research Foundation of the Franklin Institute. t Condensed from a paper published in full in the Rew;ew of Scientific Instruments, 9, I75 (I938)-233