Patent GB-1337473-A (GB)
Title: Display device for an electric signal
IPC: G01R19/155
Filed: 1971-05-25
Abstract: 1337473 Voltage measuring BROWN BOVERI & CO Ltd 25 May 1971 [27 May 1970] 16866/71 Heading G4H [Also in Division G1] A voltage measuring device of the type in which a rectangular strip of electro-optic medium (i.e. a medium, such as a liquid crystal or a photo-luminescent material, which abruptly changes one of its optical parameters when it is subjected to an electric field above a threshold level) is sandwiched between two electrodes across one of which a voltage gradient is produced in the direction of the length of the strip and to the other of which the voltage to be measured is applied, whereby the length of that portion of the strip in which the optical parameter has the value associated with an electric field above the threshold level represents the magnitude of the voltage to be measured, is characterized in that one of the electrodes is shaped like a ladder as shown in Fig. 1, each rung of which extends parallel to the interface between the two values of the optical parameter and comprises three segments, a top segment 28n<SP>111</SP> of photosensitive material whose resistance falls when it is illuminated, a centre segment 28n<SP>11</SP> of low resistance, and a bottom segment 28n<SP>1</SP> of fixed resistance, the arrangement being such that the resistances of the top segments 28n<SP>111</SP> are affected by the value of the optical parameter in such a way as to make the transition between the two values of the parameter sharper. In the preferred embodiment, Fig. 2, a strip of liquid crystal 3 of the type which either transmits plane polarized light, from a source 43 and polarizer 42 to a second polarizer 41 crossed with respect to the first polarizer 42 so that no light reaches an observer 44, or, when it is subjected to a sufficiently high electric field, scatters the plane polarized light 42, 43, so that light does reach the observer 44, is enclosed between a rear electrode 2 and a front electrode 1, the front electrode being flexible and being bent as shown so that the upper portions 28n<SP>111</SP> of each rung 28n are positioned in front of the polarizer 41. In this way, the upper portions 28n<SP>11</SP> of the rungs are either illuminated or not depending upon whether the liquid crystal scatters light or not. The system operates as follows. A linear potential gradient, from left to right, is applied to the rear electrode 2. The front ladder electrode has two voltages applied to it, a voltage V 1  = V T  + V m , where V T  is the threshold voltage of the liquid crystal and V m  is the voltage to be measured, to the bottom ends of every rung, see Fig. 1 and a voltage V 1  + #V to the top ends of every rung. Each rung acts as a potential divider, and the potential of its centre portion-which is the potion that affects the liquid crystal medium-will either be high (approximately equal to V 1  + #V) or low (approximately equal to V 1 ) depending upon whether the top portion 28n<SP>11</SP> of the rung is respectively illuminated or not, and the sharpening of the transitiion from light to no light comes about because the presence of light raises the potential of the centre portion 28n<SP>11</SP>, which rise in turn tends to cause even more light. Thus, once a rung is partially illuminated, because of the positive feedback action, it rapidly becomes fully illuminated. The system described may also include a heating arrangement (Fig. 4, not shown) to maintain the temperature of the liquid crystal constant, and an arrangement comprising an additional two rung electrode 29a, 29b, Fig. 1, to which is applied potentials which should cause only rung 29b to be illuminated. If, for any reason this is not so, the potentials are varied until it is, and corresponding variations are made to the bias potentials applied to the main indicator.