in order that the wire forming the winding may be thoroughly insulated from the core.  One end of the wire is then passed through a hole in one of the spool heads or washers, near the core, and the wire is then wound on in layers.  Sometimes a thickness of paper is placed around each layer of wire in order to further guard against the breaking down of the insulation between layers.  When the last layer is wound on, the end of the wire is passed out through a hole in the head, thus leaving both ends projecting.

[Illustration:  Fig. 98 Construction of Electromagnet]

Magnet Wire.  The wire used in winding magnets is, of course, an important part of the electromagnet.  It is always necessary that the adjacent turns of the wire be insulated from each other so that the current shall be forced to pass around the core through all the length of wire in each turn rather than allowing it to take the shorter and easier path from one turn to the next, as would be the case if the turns were not insulated.  For this purpose the wire is usually covered with a coating of some insulating material.  There are, however, methods of winding magnet coils with bare wire and taking care of the insulation between the turns in another way, as will be pointed out.

Insulated wire for the purpose of winding magnet coils is termed magnet wire.  Copper is the material almost universally employed for the conductor.  Its high conductivity, great ductility, and low cost are the factors which make it superior to all other metals.  However, in special cases, where exceedingly high conductivity is required with a limited winding space, silver wire is sometimes employed, and on the other hand, where very high resistance is desired within a limited winding space, either iron or German silver or some other high-resistance alloy is used.

Wire Gauges.  Wire for electrical purposes is drawn to a number of different standard gauges.  Each of the so-called wire gauges consists of a series of graded sizes of wire, ranging from approximately one-half an inch in diameter down to about the fineness of a lady’s hair.  In certain branches of telephone work, such as line construction, the existence of the several wire gauges or standards is very likely to lead to confusion.  Fortunately, however, so far as magnet wire is concerned, the so-called Brown and Sharpe, or American, wire gauge is almost universally employed in this country.  The abbreviations for this gauge are B.&S. or A.W.G.

TABLE III

Copper Wire Table

Giving weights, lengths, and resistances of wire @ 68 deg.  F., of Matthiessen’s Standard Conductivity.

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