xviii
INTRODUCTION
upon an experimental investigation on the formation of residual
charge in insulators. He did not seem well satisfied with the
result; at any rate he did not consider it worth writing out.
It was only by v. Helmholtz's special request that he was
subsequently induced to give an account of this research at a
meeting of the Physical Society of Berlin on 27th May
1881. It did not appear in Wiedemann's Annalen (XIV. in
this volume) until three years later, after the quantitative data
had been recovered by a repetition of the experiments made
for this purpose at Hertz's suggestion.
Soon afterwards, in October 1880, Hertz became assistant
to v. Helmholtz. He now revelled in the enjoyment of the
resources of the Berlin Institute. He was soon engaged, in
addition to the duties of his office, upon many problems both
experimental and theoretical; and expresses his regret at not
being able to use all the resources at his disposal, and to
solve all the problems at once. At this time he sowed the
seeds which during his three years' term as assistant developed
one after the other into the investigations which appear as
IV.-XVI. in this volume.
He was first attracted by a theoretical investigation “On
the Contact of Elastic Solids" (V.) During the frequent
discussions on Newton's rings in the Physical Society of
Berlin it had occurred to Hertz that although much was
known in detail as to the optical phenomena which takes place
between the two glasses, very little was known as to the
changes of form which they undergo at their point of contact
when pressed together. So he tried to solve the problem and
succeeded. Most of the investigation was carried out during
the vacation of Christmas 1880. Its publication, at first in
the form of a lecture to the Physical Society (on 21st January
1881), was at once greeted with much interest. A new light
had been thrown upon the phenomena of contact and pressure,
and it was recognised that this had an important and direct
bearing upon the conduct of all delicate measurements. For
example, determinations of a base-line for the great European
measurement of a degree were just then being calculated
out at Berlin. The steel measuring-rods used in these deter-
minations were lightly pressed against each other with a
glass sphere interposed between them. This elastic contact