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
the
way.[8]
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
earlier.[2]
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
1
1
v2
c2
---- -–
------------------ 1 =
1
2
-- -
v2
c2
---- -+
v2
c2
---- -
K
K
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