Electrolytic apparatus having a stable reference electrode and method of operating such apparatus

DETAILED DESCRIPTION Before describing the detailed arrangement and control of reference electrodes in an electrochromic display according to the invention, a basic method for writing and erasing such displays will first be discussed in connection with FIGS.
1 and 2.
The cell schematically illustrated in FIG.
1 contains a solution of an electrochromic substance such as viologen, a preferred form being a mixture of 1,1' di-heptyl-4,4'- bipyridinium phosphate and hypophosphite as described in published European patent application No.
0001912.
Within the cell are shown three electrodes, a display electrode 10, a counter electrode 11 and a reference electrode 12.
In practice, the display electrode 10 is one of a number of display electrodes which are selected as picture elements (pels) in accordance with the information to be displayed.
However, only one such electrode is shown for ease of explanation.
A preferred form of display electrode for a viologen electrochromic system is a rough plated silver electrode.
The rough silver acts as a diffuser of light and has a matt white appearance when unwritten.
The rough surface also has electrochemical advantages as explained in published European patent application No.
0004548.
The preferred counter electrode for this type of system is a platinum black plated foil at one side of the display cell.
The reference electrode 12 is an electrical conductor which is assumed to have a stable potential with respect to the solution.
Preferably the reference electrode will be of the same material as the display electrodes, in this case silver.
The contrast achievable with such a display is porportional to the electric charge passed.
To ensure uniformity of color, a constant current writing method is employed.
Thus, to write display electrode 10, the counter electrode 11 is connected to a source of potential +V and a constant current source 13 is connected to the floating display electrode 10 via switch 14 for a predetermined time period.
The passage of a predetermined amount of charge results in the reduction of a corresponding amount of viologen to its colored radical cation state at the electrode 10