Hawkins electrical guide ... : questions, answers & illustrations : a progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications ; a practical treatise (1914)
Hawkins, N. (Nehemiah), 1833-1928 · New York : T. Audel & Co. · leaf 36
hat is to say, the emergent rays are parallel. When this condition obtains, theintensity of light decreases slowly, thus, a small lamp can illuminate considerable distance. 2,690 HAWKINS ELECTRICITY may be exposed to the suns rays so that they are parallel toits axis. The emergent pencil being… … Fig. 3,795.—Experimental determination of the principal focus of a double concave lens. Theface AB is covered with an opaque substance, such as lamp black, two small apertures, Aand B, being left in the same principal section and at an equal distance from the axis.A pencil of sunlight is then…
Electrical engineering
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