# induced voltage with coil between 2 magnets

by davenn
Tags: coil, induced, magnets, voltage
 PF Patron Sci Advisor P: 1,956 hi gang, Just building a seismometer I have 2 x 2.5cm disc rare earth magnets spaced far enough apart for a coil of wire to move between them. Would you expect there to be any difference in induced voltage into a coil between 2 magnets when the magnets are orientated to attract or repel ? I'm thinking no difference, but really have no idea see pic for idea of what I'm doing. ignore that fact the builder of shown unit is using 4 x disc magnets, 2 top and 2 bottom. if there is a difference between the two options .... why ? cheers Dave Attached Thumbnails
 P: 139 Well.. can't see any picture. Got link to it ?
 PF Patron Sci Advisor P: 1,956 GRRR sorry about that I posted it and saw the pic, thought nothing more of it then after your comment in the email, I visited the page and saw the pic was missing Should be ok now :) cheers Dave
P: 139

## induced voltage with coil between 2 magnets

I'm not very familiar with that kind of stuff.. but.. from general elekctromagnetics:

I take that magnets are on opposite sides of coil and when there is earthquake you have some coil-magnets movement, correct ?
If yes then there are significant diffrences for diffrent magnets setups. You can observe signal because there is some movement between magnets and coils. Magnetic flux changes and this induces voltage in coil. The essence here is the magnetic flux generated by magnets. If they are set up in the same side N-S-N-S (or S-N-S-N) the flux is flowing, but if You set them up like N-S-S-N (or S-N-N-S) the magnetic flux will be near zero (it's like two sources set opposite). No flux = no signal.
PF Patron
P: 1,956
 Quote by gerbi I'm not very familiar with that kind of stuff.. but.. from general elekctromagnetics: I take that magnets are on opposite sides of coil and when there is earthquake you have some coil-magnets movement, correct ? If yes then there are significant diffrences for diffrent magnets setups. You can observe signal because there is some movement between magnets and coils. Magnetic flux changes and this induces voltage in coil.
yes, thats correct. Some guys use horseshoe magnets. But strong and physically relatively small ones are not readily available. So the other main choice these days are to use several rare earth disc magnets.

 The essence here is the magnetic flux generated by magnets. If they are set up in the same side N-S-N-S (or S-N-S-N) the flux is flowing, but if You set them up like N-S-S-N (or S-N-N-S) the magnetic flux will be near zero (it's like two sources set opposite). No flux = no signal.
Just for your info.... at the other end of the "red" assembly there are more magnets mounted. They have a piece of aluminium between them. This is used for dampening of the pendulum arm, so that it doesnt go into free natural oscillation. Else you are just recording the motion of the pendulum and not the earth.
Tecnically its the frame of the seismometer that is moving, not the pendulum, due to the large mass on the end of the pendulum arm. but because the pendulum isnt totally isolated from the frame it will start to oscillate as well.