6.
Note that these electrodes can be connected to an external circuit by
means of the platinum wire extensions located at the bottom of the tubes.
The porous spacers which are shown in Figure 3-1 are used to separate the
electrolyte and the negative electrode.
Additional spacers separate the
mercurous sulphate paste from the positive electrode and the electrolyte.
The mercurous sulphate paste is the depolarizer for the cell.
These two
types of cells have different properties and different uses.
7.
The saturated cell is called the "normal" cell to distinguish it from
the unsaturated type and to indicate that it is the basic standard. These
cells, when made of purified materials, show a high degree of uniformity.
Individual cells have differences, in general, of a few microvolts from the
average of a group, although an occasional cell may be much further out of
line. The EMF changes somewhat during the first few years, and then settles
to a comparatively steady value.
These cells have the properties of
reproducibility and permanence to a quite satisfactory degree.
8.
There is an acid form of the cell in which the electrolyte contains
added sulfuric acid.
This cell appears to be better with respect to
uniformity and permanence. The National Physical Laboratory in England uses
it as a standard, and the National Bureau of Standards in the United States
has a group of cells of this type in addition to the so-called neutral type.
9.
The normal cell has a rather large temperature coefficient, about -57
V/C at 30C.
In fact, at 30C, it has an effect of approximately +314
V/C at the positive terminal and 374 V/C at the negative terminal, so,
it is important to keep the entire cell at uniform temperature to secure as
much cancellation as possible of the plus and minus effects.
These cells
are always used with close temperature regulation, usually in an oil or air
bath held constant to better than 0.01C. Close regulation is needed; first
because of the large temperature coefficient and second, because this type
of cell is used only where high precision is required.
Normal cells
accommodate themselves slowly to changes in temperature and show hystersis
when the cell is heated or cooled.
The term "hystersis" as used here
represents the lag in EMF and generally erratic behavior of the cell while
it is coming to equilibrium following a change of temperature. One case has
been reported where cells were subjected to high temperature accidentally,
and required 2 or 3 months to recover.
10. The saturated cell is a standard for the maintenance of the volt, and
as such is only used in places such at the Bureau of Standards and the
largest commercial and military laboratories.
11. The unsaturated cell as stated before is similar to the normal or
saturated cell except that it contains no CdSO4 crystals, and the solution
is so constructed that it will become saturated at 4C.
Hence, it is
unsaturated at normal room temperatures. In addition, a septum or retaining
member is used over each electrode to hold the material in place against
ordinary motion so that the cell is portable.
(After shipment, the cell
should always be allowed a few days to settle.)
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