plates or sheets presented a very large surface. On the outside of the
tank, G, were also diffusers, _h_ _h'_ ... arranged all round and
presenting an appearance as if two books were open so as to form a
square with their covers, the leaves being the diffusers. The
diffusers on the outside were at right angles to those inside.

[Illustration: CROSS SECTION OF JEHL AND RUPP'S CURRENT METER.]

The action of the meter was thus: When a current passes through the
coil, R, it heats the liquid at the place, thus causing a circulation,
the warm liquid ascending while the cold liquid descends as shown by
the arrows. This circulation causes the undershot wheel to revolve,
and its revolutions are registered by the clockwork. The stronger the
current, the more the heat, and thus the more rapid the circulation.
The warm liquid once in the tank, which is of a reasonable size, will
impart its heat to all the diffusers. The surface of the glass tube,
etc., is very small in comparison to the surface of the tank. It will
be seen that the function of this apparatus is independent of the
outward temperature, for the motion of the liquid is due only to that
heat which is generated by the current. When the current does not
pass, it is evident that the liquid, at whatever temperature it may
be, does not circulate, as all parts are of the same temperature; but
the moment the current passes, a difference is produced, which causes
a circulation in proportion to the current. We may mention that we
tried various liquids, and give preference to pure olive oil. It will
also be seen that this meter is good for alternating currents. In
conclusion, we may remark that the tests we made gave satisfaction,
and we wanted to publish them, but that Mr. Jehl was called away to
fit up the Edison exhibit in the Vienna exhibition for the Societe
Electrique Edison of Paris. After the exhibition we began our work