A new mass spectrometer of the omegatron type has been designed and tested. This new device, which we have called the resotron, combines the extremely low permanent outgassing and pumping rates of the omegatron (less than 10-11 torr liter/sec. and 10-3 liter/sec. respectively) with a high scanning rate (typically 10 spectra/sec.) allowing the spectrum to be displayed on an oscilloscope. The sensitivity is about 10-12 torr of partial pressure and the resolving power (at 5% peak height) is about 40 at mass 40 for B = 4000 Gauss, or 70 at mass 70 for B = 12000 Gauss. In this paper, the performances of the omegatron and the resotron are compared. An original theory of the cyclotron resonance is developed. Based upon a circular motion approximation, this general treatment is applied to the case of the perturbation caused by the space charge of the beans, and for the conditions of rapid scanning over the resonance frequency. Experimental data are compared with the theoretical predictions. A brief description of the electronics is also given.