Quantum measuring noise at the observation of a particle in a magnetic field

Quantum theory of continuous measurements based on the Feynman integration in finite limits is applied to the problem of watching the coordinates of a particle moving in the magnetic field. Probability of a given result of the watching at a given external force is found as well as, inversely, the accuracy of finding the external force corresponding to the given measurement result, is estimated. In these calculations, the spreading of the result due to the measuring device errors is taken into account as well as the quantum-mechanical influence of the device on the motion of the partical.Three characteristic measurement regimes are singled out, the classical, quantum and optimal ones