|Institution:||University of Canterbury|
|Full text PDF:||http://hdl.handle.net/10092/8065|
Modern theory regards light as an electromagnetic disturbance and a change in electrification as the removal or addition of electrons which are negatively charged. Hence a photo electric change can be considered as the liberation of some of these negative electrons by the action of electro magnetic waves. Hertz in 1887 found that ultra-violet light falling on a spark gap caused the discharge across that gap to be made more easy, and that the effect was proportional to the actinic quality of the light source used. In 1888, Wiedmann and Ebert proved that the phenomenon takes place at the cathode of the spark gap. The “Hallwachs Effect” was discovered in the same year (1888). W. Hullwachs found that ultra violet light falling on a body carrying a charge of negative electricity caused that body to lose its charge of negative electricity very readily, but that ultra violet light falling on a positively charged body does not cause the charge to be liberated. Here then we have a difference between positive and negative electrification. Hallwachs showed later that if a body, carefully insulated and initially free from electrical charge, be exposed to ultra violet light, it requires a positive charge. The experimental methods of studying the photo electric discharge was as follows: The brightly polished metal plate which received the illumination formed one plate of an air condenser, of which the other plate, likewise polished was positively charged by means of a battery, and the continuous currant produced was measured by means of an electrometer. For a condenser constructed of a certain metal, it was found that beyond a certain potential difference, this current became independent of the field, and this saturation current could then be regarded as a measure of the photo electric activity of that metal. It was found by Elster and Geitel that the photo electric effect was more pronounced the more electro positive the body, Rubidium for example losing a negative charge when illuminated by light from a glass rod heated to redness.