[Illustration: FIG. 3.]

[Illustration: FIG. 4.]

In order to understand how a variation in currents of this kind can be
applied in general for producing a rotary motion in the two
directions, it will only be necessary to refer to Figs. 3 and 4. The
conductors, L1, L2, and L3 communicate with the bobbins of
three electromagnets, E1, E2, and E3, whose poles are bent at
right angles to the circumference of the wheel, R. There is never but
one pole opposite a tooth. The distance between two consecutive poles
must be equal to a multiple of the pitch increased (Fig. 3) or
diminished (Fig. 4) by one-third thereof. It will be seen upon a
simple inspection of the figures that R will revolve in the direction
of the hands of a watch when the currents follow the order L1,
L2, L3, etc., in the case shown in Fig. 3, while in the case
shown in Fig. 4 the rotary motion will be in the contrary direction
for this same order of currents. But, in both cases, and this is the
important point, the direction of rotation changes when the order in
the succession of currents; is inverted. Fig. 6 gives a perspective
view of the registering apparatus, and Fig. 5 represents it in
diagram. It will be at once seen that, the toothed wheel, r, is
reduced to its simplest expression, since it consists of two teeth
only. The electro-magnets are arranged at an angle of 120°, and for a
change of current the wheel, r, describes an angle of 60°, that is
to say, a sixth of a circumference. The motion of r is transmitted, by
means of the pinion, d, and the wheel, e, to the wheel, T. For a
one-meter variation in level the wheel, T, makes one complete
revolution. It is divided into 100 equal parts, and each arc therefore
corresponds to a difference of one centimeter in the level, and