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HANDBOOK OF KIN E M AT 0 GRA PH Y. 157 CHAPTER IV. THE ILLUMINANT. ELECTRICITY. Now to the consideration of the illuminant to use in the projection of moving pictures. Practically speaking, except for amateur work —exhibiting films at home and such like—there are only two alter- native forms of illuminant available :—limelight and electricity. Even here our choice is still further limited by the recent Home Office regulations governing the conditions under which limelight may be employed. Comparing the two forms of illuminant on the grounds of economy and efficiency, there can be no doubt as to the verdict for all serious projection work lying with electricity. As to efficiency, electricity can be made to give an amount of illumination only limited by our expenditure upon current, whereas limelight, apart from every consideration of economy, can never do more than yield a light rather under than over three thousand candle power. Further, the light source with electricity is far smaller than that obtained from the glowing lime, with the result that in the former case the concentra- tion of the pencil upon the projector gate is correspondingly more free from wasteful nebulous edge. Further, electric light for projection purposes is somewhat easier to manage than limelight (both require skilled handling) and on the score of economy may prove anywhere up to ten times as cheap in use, especially where the kinematograph show is remote from the nearest centre for gas cylinder charging, and where electric current is produced on the spot by means of a good class private generating plant. For the foregoing reasons, electricity claims first place in our description of kinematograph illuminants. We will start upon its manner of employment in the projection arc by giving a brief sketch of its generation and measurement, also some of the most important terms of nomenclature. Electricity, or electric current, as it may be called, can be generated in various ways. Moreover, the manifestation of it is often vastly different according to how it is called into being. For instance, lightning and the commodity which produces rotation in an electric motor are both electricity, and the Only reason for the vastly different phenomena connected with the two manifestations of electrical energy is to be found in the manner of the current's generation. The form of electricity which alone comes into evidence as the electrical supply of a picture hall is a continual " flow " or " current " manufactured by means of a machine called a dynamo or generator. Such a generator consists of a soft iron drum wound about with coil upon coil of insulated copper wire and made to rotate at very high speed within the field of a powerful magnet. This rotating