"Each equalizer has a bore in it half-filled with mercury, working a good deal like a carpenter's level.  If the airplane tilts to one side or the other, the mercury will try to keep its level and will immediately flow to the high side of the bore.  At each end of this mercury tube there are electrical contact points.  As one becomes submerged in the mercury by a tilting of the plane, a connection is made whereby two electro-magnets are energized on that side.  One of these magnets closes an exhaust-valve, and the other opens an inlet-valve, in the compressed air tank.  At once air is forced into this double cylinder, which you see at the bottom of the stabilizer, filling the half which is to operate its own set of rudders; and a piston begins to work inside.  The piston is connected to a toothed rack, as you will note, causing this to turn a sector engaging it.  The control wires connect with this sector."