see later that this is exactly the same procedure employed in D.C. socket-powered receivers, where the filaments are connected in series.
Some of this may seem rather far from Radio, but the Radio-Trician should know considerable about the simple problems of general electricity. A large portion of Radio receivers receive their operating current from city lines and the Radio-Trician must make sure, not only that the receiver is connected properly to the source of supply but that the proper voltage is being obtained for best operation. A fundamental knowledge of electrical wiring often enables Radio Tricians to prevent serious damage to Radio receivers or other electrical apparatus.
It is never wise to connect any piece of apparatus to a source of supply until you know that the voltage and current requirements are met. For example, electricity for lighting purposes within a radius of about a mile from a generating station is
Fig. 13(a)
generally direct current. A Radio-Trician who knows his business will never allow an A.C. receiver to be connected to a D.C. line.
Current supplied to points more than a mile from the generating station is ordinarily alternating current and of course, the Radio-Trician who knows his electricity would not try to operate a D.C. receiver from alternating current, or for that matter any piece of apparatus which was designed for D.C. operation.
Before leaving the subject of electrical power systems, it will be well to consider briefly an ordinary three-wire transmission system. You may have noticed three wires entering a house and you may have wondered why there were three wires instead of two. As previously mentioned, one wire is common so that in a three-wire system we really have two circuits each of which uses a common return wire. This common wire is often called the neutral wire.
