The photo-electric behaviour of iron in the active and passive state

The discoveries of Hertz and of Hallwachs showed that a polished metal plate exposed to ultra-violet light readily loses a negative electric charge, but retains a positive charge. If an electric Held is applied by making the metal form one plate of an air condenser, whose second plate is charged positively by a battery, the saturation current obtained may be regarded as a measure of the photo-electric activity of the metal. It has been known for more than a century that ordinary iron, which is acted on energetically by dilute nitric acid, can be made to assume a passive condition by immersion in strong nitric acid. The same condition can be produced by other powerful oxidising agents, or by using iron as the positive electrode in an electrolyte containing oxygen. The nature of the change that takes place when iron passes from the active to the passive state has given rise to much discussion, but none of the explanations suggested has yet met with general acceptance.