In the matter of heat-resisting properties the enameled wire possesses a great advantage over silk and cotton.  Cotton or silk insulation will char at about 260 deg.  Fahrenheit, while good enameled wire will stand 400 deg. to 500 deg.  Fahrenheit without deterioration of the insulation.  It is in the matter of liability to injury in rough or careless handling, or in winding coils having irregular shapes, that enamel wire is decidedly inferior to silk or cotton-covered wire.  It is likely to be damaged if it is allowed to strike against the sharp corners of the magnet spool during winding, or run over the edge of a hard surface while it is being fed on to the spool.  Coils having other than round cores, or having sharp corners on their spool heads, should not ordinarily be wound with enamel wire.

The dielectric strength of enamel insulation is much greater than that of either silk or cotton insulation of equal thickness.  This is a distinct advantage and frequently a combination of the two kinds of insulation results in a superior wire.  If wire insulated with enamel is given a single wrapping of silk or of cotton, the insulating and dielectric properties of the enamel is secured, while the presence of the silk and cotton affords not only an additional safeguard against bare spots in the enamel but also a certain degree of mechanical protection to the enamel.

Winding Methods.  In winding a coil, the spool, after being properly prepared, is placed upon a spindle which may be made to revolve rapidly.  Ordinarily the wire is guided on by hand; sometimes, however, machinery is used, the wire being run over a tool which moves to and fro along the length of the spool, just fast enough to lay the wire on at the proper rate.  The movement of this tool is much the same as that of the tool in a screw cutting lathe.

Unless high voltages are to be encountered, it is ordinarily not necessary to separate the layers of wire with paper, in the case of silk-or cotton-insulated magnet wire; although where especially high insulation resistance is needed this is often done.  It is necessary to separate the successive layers of a magnet that is wound with enamel wire, by sheets of paper or thin oiled cloth.

[Illustration:  Fig. 99.  Electromagnet with Bare Wire]

In Fig. 99 is shown a method, that has been used with some success, of winding magnets with bare wire.  In this the various adjacent turns are separated from each other by a fine thread of silk or cotton wound on beside the wire.  Each layer of wire and thread as it is placed on the core is completely insulated from the subsequent layer by a layer of paper.  This is essentially a machine-wound coil, and machines for winding it have been so perfected that several coils are wound simultaneously, the paper being fed in automatically at the end of each layer.

Another method of winding the bare wire omits the silk thread and depends on the permanent positioning of the wire as it is placed on the coil, due to the slight sinking into the layer of paper on which it is wound.  In this case the feed of the wire at each turn of the spool is slightly greater than the diameter of the wire, so that a small distance will be left between each pair of adjacent turns.