ating device. We also remember in the year 1873 or
1874, when his mind was occupied with his "Standard turbine," he was
hindered by some device used now on locomotives of the present
construction (what it was we are unable to say), but when draughting
at his water wheel, would conflict the two, and by his invitation we
wrote to a prominent locomotive builder and had him examine the
drawings, which he had not fully completed, and sold same to him. Of
this we only have a faint recollection, but do recollect his saying:
"Well, that is off my mind now, and I can devote it to the finishing
of my new wheel."--_American Miller_.

* * * * *

ALTERNATE CURRENT CONDENSERS.

At a recent meeting of the Physical Society, London, Mr. James
Swinburne read a paper on alternate current condensers. It is, he
said, generally assumed that there is no difficulty in making
commercial condensers for high pressure alternating currents. The
first difficulty is insulation, for the dielectric must be very thin,
else the volume of the condenser is too great. Some dielectrics 0.2
mm. thick can be made to stand up to 8,000 volts when in small pieces,
but in complete condensers a much greater margin must be allowed.
Another difficulty arises from absorption, and whenever this occurs,
the apparent capacity is greater than the calculated. Supposing the
fibers of paper in a paper condenser to be conductors embedded in
insulating hydrocarbon, then every time the condenser is charged the
fibers have their ends at different potentials, so a current passes to
equalize them and energy is lost. This current increases the capacity.
One condenser made of paper boiled in ozokerite took an abnormally
large current and heated rapidly. At a high temperature it gave off
water, and the power wasted and current taken gradually decreased.

When a thin plate of mica is put between tin foils, it heats
excessively; and the fall of potential over the air films separating
the mica and foil is great enough to cause disruptive discharge to the
surface of the mica. There appears to be a luminous layer of minute
sparks under the foils, and there is a strong smell of ozone. In a
dielectric which heats, there may be three kinds of conduction, viz.,
metallic, when an ordinary conductor is embedded in an insulator;
disruptive, as probably occurs in the case of mica; and electrolytic,
which might occur in glass. In a transparent dielectric