Reed Switch Operation:
form a: single pull-single throw (spst), normally open (no)
In terms of reed switch operation, this is the most common reed switch configuration. The Form A switch has two contacts that are Normally Open, meaning that the introduction of a magnetic field will cause the contacts to close, which closes the circuit, thus conducting electricity. Upon removal of the magnetic field the contacts will return to the Normally Open state, opening the circuit and stopping the flow of electricity. A Form A switch can be wired without regard for the direction of electrical flow. reed switch operation
form b: single pull-single throw (spst), normally closed (nc)
This is the least common reed switch configuration and operates the opposite of a Form A. The Form B switch also has two contacts, but they are Normally Closed, meaning that, in its default state, the circuit is closed and will conduct electricity. Introduction of a magnetic field will cause the contacts of a Form B switch to separate, opening the circuit, breaking the flow of electricity. Removing the magnetic field will allow the contacts to return to Normally Closed thereby closing the circuit. A Form B switch can also be wired without regard for the direction of electrical flow. reed switch operation
form c: single pull-double throw (spdt), normally open/normally closed (no/nc) reed switch operation
The Form C reed switch configuration is also quite common. A Form C switch has three contacts, a Common and one each, Normally Open and Normally Closed. Essentially, a Form C switch has the capability of both a Form A and Form B switch in a single package. In its default state, electricity flows from the Common contact through the Normally Closed contact. Upon introduction of a magnetic field, the Common contact moves from the Normally Closed contact to the Normally Open contact, opening one circuit and closing the other, thus redirecting the flow of electricity. Removal of the magnetic field will cause the Common contact to revert back to its initial Normally Closed position. Unlike either the Form A or Form B switch, direction of electrical flow should be considered when wiring a Form C switch, for typical SPDT functionality the electrical source would be attached to the Common lead. That is not to say that a configuration where switching between two electrical sources on the NO and NC leads to the Common lead is not possible, it simply is not the norm.