Polarized Ringers or Bells

In these figures, c and c₁ are soft-iron cores connected at the upper end by a soft-iron strap s-s₁ This strap also serves to hold the gongs g and g₁. The soft-iron armature a-a₁ is pivoted as indicated and has attached to it the ringing device. Two high-impedance windings w and w₁ are placed on the cores as indicated. The resistance of these windings is of the order of from one thousand to several thousand ohms, and the impedance is very much higher, depending on the frequency of the current. A permanent magnet m-m₁ is placed as shown.

Figure 5. A polarized ringer.

There are, therefore, two magnetic circuits. The first of these is for the flux produced by the ringing current through the coils and is along the soft-iron core c, through the soft-iron strap s-s₁, through the other core c₁, across the air gap to the armature a₁, along the armature to a, and then back across the air gap at a to the core c. The other path is for the magnetic flux produced by the permanent magnet m-m₁. This path is from the lower end of this permanent magnet across the large air gap to the center of the armature a-a₁ where the flux divides, half of it passing through the armature and across the air gap at a, and the other half passing through the armature and across the air gap at a₁ Each half of the flux produced by the permanent magnet then passes through the coil c or c₁ and through the strap s-s₁ to the end of the permanent magnet.

The operation of the bell can be explained in the following manner. Suppose the magnet is poled so that it gives the ends of the armature a-a₁ a south polarity. With no current flowing, no unbalance will exist and the armature will occupy the balanced position shown. When an alternating ringing current flows through the bell, flux wall be produced by the windings. Assume that the current at a particular instant has the direction indicated by the arrow; then, the flux φ produced by this current will have the directions indicated and will tend to make the end of core c₁ a stronger north pole, and c a weaker north pole. The armature will therefore be unbalanced and will be attracted to the pole c₁, and the bell will ring. When the opposite part of the alternating current passes through the winding, the action will be reversed and the bell will again ring.