The comparative slowness of selenium in responding to
any great changes in the illumination offers a serious difficulty to its use in photo-telegraphy, but various methods have been devised whereby the effects of inertia can be counteracted. In the system of De' Bernochi (see Chapter I.) the changes in the illumination are neither very rapid nor very great, and the inertia effects would therefore be very slight; but in any photo-telegraphic system in which a metal line print is used for transmitting, where the source of illumination is constant and the resistance of the cell is required to drop to a definite value and return to normal instantly, many times in succession, the inertia effects are very pronounced. The most successful method of counteracting the inertia is that adopted by Professor Korn of always keeping the cell sufficiently illuminated to overcome it, so that any additional light acts very rapidly. Another method worked out and patented by Professor Korn, and known as the "compensating cell" method, gives a practically dead beat action, the resistance returning to its normal condition as soon as the illumination ceases. The arrangement is given in the diagram Fig. 56.
Light from the transmitting or receiving apparatus, as the case may be, falls upon the selenium cell S1, which is
placed on one arm of a Wheatstone bridge, a second cell S2 being placed on the opposite arm. The selenium cell S1 should have great sensitiveness and small inertia, the compensating cell S2 having proportionally small sensitiveness and large inertia. Two batteries B, B', of about 100 volts, are connected as shown, B being provided with a compensating variable resistance W; W' is also a regulating resistance. When no light is falling upon the cell S1, light from L is prevented from reaching the second cell S2 by a small shutter which is fastened to the strings of the Einthoven galvanometer (described in Chapter III.), and the piece of apparatus C—relay or galvanometer as the case may be—remains in a normal condition. When, however, light falls upon the cell S1, the balance of the bridge is upset, and light from L falls a fraction of a second later upon the second cell S2, and the current flowing through C completes the circuit. Needless to say it is necessary that the two cells be well matched, as it is very easy to have over-compensation, in which case the current is brought below zero.
It is also stated that by enclosing the cells in exhausted glass tubes, their inertia can be greatly reduced and their life considerably prolonged. The sensitiveness of a cell is the ratio between its resistance in the dark and its resistance when illuminated. The majority of cells have a ratio between 2:1 and 3:1, but Professor Korn has shown mathematically that by conforming to certain conditions regarding the construction the ratio of sensitiveness may be between 4:1 and 5:1. Thus a cell of R = 250,000 ohms can be reduced to 60,000 ohms from the light of a 16 c.p. lamp placed only a short distance away; the resistance may be still
further decreased by continuing the illumination, but this produces a permanent defect in the cells termed "fatigue," the cells becoming very sluggish in their action and their sensitiveness gradually becoming less, the ratio between their resistance in the dark and their resistance when illuminated being reduced by as much as 30 per cent.