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CHAPTER \ I RECTIFIERS
(1) Power supply to most American projection rooms is ax. but every projection room requires d.c. Eor opera
tipn of important items in its equipment. The majority of projection light sources are d.c. arc lamps. All sound amplifiers, without exception, require (I.e., even those that are "a.c. operated." Only direct current can be used for the plates and grids of such tubes a shown in Figs. 42 and 43. Hence, alternating current supplied by the power line must be converted to direct current before it can Inused for certain projection room purposes.
(2) Conversion of a.c. to d.c. is known as rectification. Apparatus that effects this conversion is usually called a "rectifier." One or more rectifiers will be found in every projection room supplied with a.c. power. In a.c. operated sound amplifiers the rectifier is built into the amplifier unit and constitutes an integral part of it.
(3) Rectification can be effected by mechanical means. by electrical means, or by a combination of both. A special form of mechanical rectifier, the "commutator," which is an integral part of d.c. generators, will be con sidered in another place (see Page 130). It is highly im portant in projection room practice, but the most common rectifying devices are electrical and the word "rectifier" usually is applied only to contrivances purely electrical in their nature.
(4) All electrical rectifiers depend for their action upon what is called uni-lateral, or one-wav conductivity. An example of a one-way conductor of a type commonly used in projection rooms is given in Figs. 40 and 41. In those tubes, as was explained, electrons are emitted by the cathode and can pass through the vacuum or gas of the tube to the plate; but there is no appreciable emission of
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