V O L U M E 5 , D O C U M E N T 2 3 5 a 5
of the epistemological foundations of the theory of relativity, to which I obligated
myself by giving an uncautious promise. I did not do a good job with the thing by
With best greetings your devoted
A. Einstein
My wife sends best greetings, too.
Vol. 5, 203a. To Vladimir Varic;ak
[Zurich, 23 April 1910]
[Not selected for translation.]
Vol. 5, 235a. To [Otto Lehmann][1]
Zurich, 1 December 1910
Highly esteemed Colleague,
First of all, many thanks for your kind letter, your paper, and above all for the
papers you sent me
Now to your example!
1) Your consideration must still take into account that the lines of force emanat-
ing from a rod become denser as a result of the Lorentz contraction. The electric
field strength is thus increased by the ratio , whereby the
electrostatic energy is increased by this ratio.
2) The relative lessening of the repulsion through electrodynamic forces that you
observed has the value ; it is therefore only halfway compensated by the in-
creased repulsion mentioned under (1). The energy is therefore in fact larger
smaller than if the rods were at rest.
3) This does not indicate a violation of the principle of relativity, however, be-
cause with moving rods one must distinguish between the force K, acting between
them, from the standpoint a frame of reference not moving along with them, and
the force , from the standpoint of a frame of reference moving along with the
rods. If differed from the force between the rods at the same distance while they
were at rest, there would be a contradiction with the principle of relativity. But the
force K between the rods from the point of view of the non-moving system can
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