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A priori it is quite clear that we must be able to learn

something about the physical behaviour of measuring-rods and

clocks from the equations of transformation, for the magnitudes

z, y, x, t, are nothing more nor less than the results of

measurements obtainable by means of measuring-rods and clocks. If

we had based our considerations on the Galileian transformation

we should not have obtained a contraction of the rod as a

consequence of its motion.

Let us now consider a seconds-clock which is permanently situated

at the origin (x′ = 0) of K′. t′ = 0 and t′ = 1 are two

successive ticks of this clock. The first and fourth equations of

the Lorentz transformation give for these two ticks:

t = 0

and

image011

As judged from K, the clock is moving with the velocity v; as

judged from this reference-body, the time which elapses between

two strokes of the clock is not one second, but

image012