the magnets outside the coil are just acting as a convenient support 
column for the ones that are inside.  I'll start thinking about how to 
get some into the other side of the coil...

Ian

Dave Nelson wrote:

>
> Agree with you John,
>      Ian,  as shown in your drawing 1/2 of your magnets are outside 
> the coil area and only one side of the supporting bar.  This severely 
> reduces the amount of voltage generated in the coils.
>
>    As to Geoffery's questions,
>             another good rule is ALWAYS use love noise components....
>
>     Bi-FET Op-Amps     ....  eg  the  OPA 027  and  similar series,  
> rather 741   etc type
>
>     Metal Film resistors  ... particularly are much lower noise 
> producing than
>                                        standard carbon resistors
>
>    I have also always used coaxial cable for feedline between the 
> sensor and the
>   preamp board.  and for the short period geophones ... Bury them !   
> even 6 inches below ground level cuts out a lot of stray RF field 
> pickup and other electrical noise.
>
> cheers
> Dave
>
>
>
> At 01:01 PM 25/03/2006 -0500, you wrote:
>
>> ian wrote:
>>
>>> just to clarify what I said about magnets, I've drawn a simple sketch:
>>> http://www.iasmith.com/pickup.gif
>>
>>
>> You might crank the signal up a bit more by splitting that stack of 
>> magnets in half, and clamping or gluing them on opposite sides of an 
>> iron slug, with the magnets repelling each other at the attachment to 
>> the slug.  The slug should sit in the center of the coil.
>>
>> The coil generates voltage in proportion to the flux that is passing 
>> radially through the coil, and this arrangement crowds a lot more of 
>> the flux in that direction.  It also makes the total magnetic field 
>> balanced, so that the Earth's magnetic field cannot push or pull on 
>> the magnets as it changes.  This reduces interference from things 
>> like solar storms.
>>
>> Your present design wasts a lot of flux that passes out the hole in 
>> the coil and generates no signal.  Making the change I am suggesting 
>> might double your signal while reducing some sources of interference 
>> and not increasing resistive noise at all.
>
>
>
>
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