The construction of two selenium bridges, for studying the electrical properties of the well known acicular crystals of this element, is described. The bridges are distinguished by the devices employed to apply the pressure necessary for good contact between the crystals and a form of metallic grid used in both. They have been designed to eliminate difficulties of insufficient sensitiveness and an unsteadiness experienced by previous workers with certain phases of the light effect.
The characteristics of the bridges are such that, together, they have a wide range of usefulness. A screw type of pressure device provides a general utility bridge, while a gravity type results in an arrangement which is more suitable for the purpose of exact measurement. When the temperature is maintained constant, variations in the dark current through the first bridge are of the order of 10−7 ampere where the potential drop is two volts. In the second bridge they may be made a hundred times smaller. The grid feature, common to both bridges, brings about a change of current as much as a thousand times that obtained by illuminating a single crystal.
A theory of the unsteadiness, observed in the electrical resistance of selenium crystals by several investigators, is presented. It is believed that a steady condition can be secured only when the pressure used to make contact with the crystals is such that their elastic limit is not reached. A number of facts, collected during the development of the bridges mentioned above, are advanced in support of this view. A practical method for complying with the condition is given.
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