The action of this rock plate is peculiar. It is pivoted at the rear
end, not to a fixed point, but to a short crank arm, the bearing for
which is inclosed in the small box shown. As the first wheel of a train
which is approaching in the desired direction (from the right in the
engraving) touches it, it will be seen that it must not only depress it,
but produce a slight forward motion, causing a corresponding rotary
motion in the rock shaft which actuates the apparatus. On the other
hand, when a train is approaching from the other direction, or has
already passed the crossing, its wheels strike first the curved plate to
the left of the illustration, and by means of the peculiar link
connections shown, depress the rock plate so as to clear the wheels
before the wheels touch it, but the depression is directly vertical, so
that it does not give any horizontal motion to it, which would have the
effect of actuating the rock shaft. Consequently, trains pass over the
apparatus in one direction without having any effect upon it whatever,
the different point at which the same force is applied to the rock plate
giving the latter an entirely different motion.
[Illustration: FIG. 2.--MAGNETO-ELECTRIC CROSSING SIGNAL]
The slight rotary motion which is in this way communicated to the rock
shaft, when a train is approaching in the right direction, compresses a
spring inside the case.
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