at a maximum the
other is zero. When such a double current is sent into a similarly
constructed motor it will produce or generate what might be called a
rotary field, which is shown diagrammatically in the six successive
positions in Fig. 2. The winding here is slightly different, but it
amounts to the same thing as far as we are concerned at present. This
is what Mr. Dobrowolsky calls an "elementary" or "simply" rotary
current, as used in the Tesla motors. A similar system, but having
three different currents instead of two, is the one used in the
Lauffen transmission experiment referred to above.

[Illustration: FIG. 1.]

[Illustration: FIG. 2.]

In investigating this subject Mr. Dobrowolsky found that the best
theoretical indications for such a system would be a large number of
circuits instead of only two or three, each differing from the next
one by only a small portion of a wave length; the larger their number
the better theoretically. The reason is that with a few currents the
resulting magnetism generated in the motor by these currents will
pulsate considerably, as shown in Fig. 3, in which the two full lines
show the currents differing by 90 degrees. The dotted line above these
shows how much the resulting magnetism will pulsate. With two such
currents this variation in magnetism will be about 40 degrees above
its lowest value. Now, such a variation in the field is undesirable,
as it produces objectionable induction effects, and it has the evil
effect of interfering with the starting of the motor loaded, besides
affecting the torque considerably if the speed should fall slightly
below that for synchronism. A perfect motor should not have these
faults, and it is designed to obviate them by striving to obtain a
revolving field in which the magnetism is as nearly constant as
possible.

[Illustration: FIG. 3.]

If there are two currents differing by 90 degrees, this variation of
the magnetism will be about 40 per cent.; with three currents
differing 60 degrees, about 14 per cent; with six currents differing
30 degrees it will be only about 4 per cent., and so on. It will be
seen, therefore, that by doubling the three-phase system the
pulsations are already very greatly reduced. But this would require
six wires, while the three-phase system requires only three wires (as
each of the three leads can readily be shown to serve as a return lead
for the other two in parallel). It is to combine the advantages of
b