2 8 D O C . 3 2 M A Y 1 9 1 9
32. From Paul Epstein
Zurich, 19 Scheuchzer St., 2 May 1919
Highly esteemed Professor,
To my dismay, my departure to Zurich suffered some delay due to passport dif-
ficulties, which made it impossible for me to benefit from your lecture
must therefore reiterate my thanks in writing for the steps that you took on my be-
half at that time. In the end I did make it to Munich, after all; all’s well that ends
Miss Edith Einstein recently approached me with the question of assisting her in
preparing a doctoral dissertation, the topic of which she received from
I nat-
urally told her that it would be a special pleasure for me. Quite en passant I had
already worked on the radiometer problem without total resolution; that is why I
am especially delighted to gain from you the viewpoints I was lacking. As Miss
Edith told me, she wrote you about a rough little calculation I had given her as a
starting point.[4] It involves the tranfer of the quantity of motion in the first moment
after heating the wall, as long as the density distribution
can be considered constant. If Iam not mistaken, one
can then directly apply Boltzmann’s generally derived
formula for the transfer of motion and obtain a radiome-
tric force independent of pressure. It is possible that this
rough theory already accounts for some of Crookes’s
experiments where the arrangement corresponded to the
accompanying sketch: L denotes the light source, A the
mica disk, B the blackened radiometer blade whose deflection was measured by a
torsion balance. That is, the first ballistic deflection after removal of the blind S.
The independence of the pressure is confirmed between 760 mm and 40 mm, but
not at lower pressures.[5]
Now, the main question is, of course, how things stand in the stationary state. I
thought that the one-sided imbalance of the velocities could be compensated by a
suitable distribution of pressure. But perhaps the assumption of circulation cannot
be circumvented either. As I said, I did not come to any resolution about that.
Recently I became acquainted with the paper by Maxwell on this problem, who
seems to assume the latter.[6] All the same, the phenomena of temperature jumps,
etc., were not yet known to Maxwell. I did not have occasion to look at Reynolds’s
In any event, Miss Einstein has enough to do for now in familiarizing herself
with these issues, so a more thorough discussion can be postponed to the time of
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