The reed switch: A simple electro-mechanical device with a contact area and two to three leads that, when the correct magnetic or electrical force is applied, facilitates continuity through the switch. Although simple in nature and application, we engineers sometimes can get a little creative when it comes to the correct application of this device. Below are three common “no-no’s” that can help you get the best performance out of your reed switch.
No-No #1: Treating a reed switch like a resistor
Whenever the reed switch leads are manipulated in any way, it can affect the magnetic properties of the switch. For example, you cannot bend or cut the leads and expect to get the same magnetic properties of the switch as when the leads are straight or at standard length. The switch is designed with specific properties. It is recommended that any custom modifications, such as bending or cutting, be done by the manufacturer, who can make sure that the resultant magnetic effect is understood and that the hermeticity is maintained in the process.
No-No #2: Dropping the reed switch or smacking it on a table
As obvious as it may sound, dropping the reed switch or even banging it up against a table--as we’re tempted to do when the switch is not operating as expected--is not a very good idea. Due to the properties within the hermetic seal, the internal workings of the switch are highly sensitive, especially in the smaller switches. If the switch does not work, replace it. Don’t break it.
No-No #3: Overloading the reed switch
Again, it may seem self-evident, yet it occurs from time to time that a switch specified to a certain load is pushed a little too far over the edge. The electrical specifications on the switch have been tested and approved so for instance, loading a 250-mA switch with 3 amps might not provide the results that you are looking for.
I hope these fundamental facts about reed switch application will prove to help you in future projects and potentially save a lot of time on R&D and troubleshooting.
Check out the original post here: http://www.hsisensing.com/resources/making-sense/m.blog/309/top-three-reed-switch-no-no-s 
The reed switch: A simple electro-mechanical device with a contact area and two to three leads that, when the correct magnetic or electrical force is applied, facilitates continuity through the switch...