We have discussed the motion of a charged particle in various configurations of electric and magnetic fields, and shall now examine some of these cases from the point of view of another observer moving at a uniform velocity u. In Chapter 3 we applied this method of examination to the dynamical behaviour of particles under the influence of such recognizable forces as are exerted by strings etc., and we now extend the argument to charged particles influenced by electric and magnetic fields. For the sake of definiteness we shall suppose that the fields are produced by charged conductors and permanent magnets at rest relative to the observer referred to in the last chapter. The moving observer therefore observes events resulting from the movement of particles in fields which are themselves in motion, in the sense that he is aware of the sources of the fields moving at velocity −u in his frame of reference. If u is not comparable to the velocity of light we may accept, in the light of the discussion in Chapter 3, that both observers agree on the measurement of mass and acceleration, but we shall quickly find that they do not agree on the origin of the force responsible for the acceleration.
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