The Differential Bell.
Sparking at the breaking contacts of an electric bell is detrimental to the platinum points, and many remedies have been devised to overcome it.
Sparking is due to the self-induction of one turn of the wire coil acting on its neighbor, and this property is utilized in the gas engine, or gas-lighting spark coil, where a fat spark is needed to ignite gas.
The differential bell has two windings in opposite directions. The action of one would be to produce an N-pole at one end and an S-pole at the other. But the second coil produces poles just the opposite, as the polarity of a magnet depends on the direction in which the current flows around it.
Where the current flows around the first winding the armature is attracted and its spring contact meets the contact screw and allows the current to divide, part flowing through the first coil, the other flowing in the reverse direction in the opposite way. One coil would tend to produce an N-pole where the other coil produced an S-pole, and these opposite poles would so neutralize each other that there would be no magnetism.
The armature would therefore be pulled back by its spring when both coils were thrown into circuit. In so doing it would cut out one coil and the same series of operations would recommence.
As a spark is normally produced where magnetism is lost by a break of circuit,[B] no spark appears, as magnetism is produced by a break of circuit in this case.
[B] For a full explanation of self-induction see No. 1 of this series.