"--_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.
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