Handy little devices.
Reed switches are a contact hermetically sealed in a glass envelope, they are usually normally open and can be closed by a magnetic field
They have a very long life expressed in billions of operations
they tend to be rated in milliamps but 2 Amps ones are quite common
if you exceed their current capacity they can seal shut and its possible to weld weld the contacts together from which there is no recovery.
I use several on my old Eldorado in the direction/Dimmer switch.
A direction/Dimmer switch on my Eldorado, the blinkers are self cancelling when you pull the clutch in
the RH switch drives a relay in the bucket to select High or Low headlight filament
front brake switch and rear brake switch.
The front brake switch is protected inside a brass tube and triggered by small magnets glued to the brake lever.
The contact closing field is expressed in Amp Turns AT 15 - 20 is common
So suppose you want to trigger the switch based on a current all you need to do is wind a coil around it e.g. 15 turns of wire for 1 Amp of current or 30 turns for 1/2 Amp etc.
A sensor like that could be used to verify a lamp is working
A switch and magnet could be used as an everlasting hermetically sealed stand switch
On my V7III I use a pair of switches inside the headlight bucket triggered thru the buckets plastic case by a small magnet on the outside to switch my 10 Watt spot lights.If I dont want the lights I just move the magnet to an adjacent piece of steel.
I used one on my VII Sport to trigger the Speedhut speedometer, a magnet spinning in the cable drive at the gearbox delivered speed pulses directly to the speedo
These little devices have a million uses.