This construction was adopted in the
first instruments made, for the purpose of observing the rate of heat
transmission through the successive compartments, but these parts are
without importance with respect to the practical use of the instrument,
and may as well be omitted, as they considerably increase the cost,
being nickel-plated and polished on both sides. The top and bottom
plates of the cover are of 0.01 inch brass, nickel-plated and polished
on both sides, both convex outward, the bottom plate but slightly, the
top plate to 4.25 inches radius. A ring of hard rubber connects, yet
separates and insulates these plates, and they are bound together with
the ring into a firm structure by a tube of hard rubber, having a
shoulder and knob at the top, and at the lower end a screw-thread
engaging with a thin nut soldered to the upper side of the bottom plate.
When the cover is in place, its lower plate is even with the top of
the cell; and the contained water, which nearly fills the cell, is
surrounded by polished, nickel-plated, brass plates 0.01 inch thick,
insulated trom other metal by interposed hard rubber. The spaces between
the cell and case (a single space if the partitions are omitted), the
space above the hard rubber rings, and the space or spaces in the cover
are all filled with eider-down, which costs $1.
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