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.