There are two different types of expansive bit, each named after the man who filed the patent for their design. These two types are Wright patent expansive bits and Clark patent expansive bits.
They are differentiated by the way the adjustable cutter is lengthened or shortened when attached to the bit.
Wright patent expansive bits
Wright patent expansive bits have adjustable cutters with teeth cut into the top.
The length of the cutter is altered by turning the adjuster screw clockwise or anti-clockwise. The screw itself turns a cog mechanism, which in turn pushes the cutter forwards or backwards using the teeth on its upper edge.
This type of bit is rarer than the Clark patent bit, and it was named after an American hardware company owner, Alfred M. Wright, when he invented it in 1939.
Clark patent expansive bits
Clark patent expansive bits have adjustable cutters with no teeth along their upper edge.
They attach to the main body of the bit via a wedge and clamp system. To adjust the bit, a DIYer loosens the lock screw and slides the cutter to the correct length manually, before tightening the screw once again.
This is the oldest type of expansive bit currently in use, and was patented in 1858 by another American inventor, William A. Clark.