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10 deduced for the transverse mass of a moving electron is in fairly good agreement with the results of Kaufmann's experiments, and has been verified very closely in a recent experiment made by Bucherer.

At present the theory is being widely used as a working hypothesis, and is of great theoretical importance, as it enables us to pass from the known analytical specification of a phenomenon for a medium at rest to the corresponding case of a medium in motion. It has been used in this way by the late Hermann Minkowski to obtain a scheme of electromagnetic relations for ponderable bodies in uniform motion, and has been shown by Planck and von Mosengeil to provide a very useful method of studying the properties of radiation in a cavity in a moving body.

With regard to the ideas of time that have arisen in connection with the theory, it may be mentioned that two new terms have been introduced. The term local time is used by Lorentz to denote time as it is measured by a set of observers who are moving uniformly in a straight line relative to a standard set of observers. The relation between the local time and the standard time is a reciprocal one, the local time for one set of observers being the standard time for the other set, and vice-versa.

A second term, used by Minkowski, is Eigenzeit or the "proper time." It is defined for each particle and may be regarded as the age of a particle. As a particle moves from one point to another, the increase of age depends upon the increase of the standard time, and also upon the velocity of the particle. If the particle is moving