Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Badger on 15/11/2007 15:41:22
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Q: What is the magnet in an electric guitar pickup for? Why is it needed? Is it there to magnetize the strings? Couldn't magnetic strings be used instead? Please read on...
Could someone please explain the dynamics of how the connected properties play out beginning with the vibrating string and ending with an alternating current in the pickup coil.
I understand the way a microphone and a speaker work. A guitar pickup has a similar set of property dynamics (if you can bunch those two words together) as a dynamic microphone: a vibrating physical body causes an alternating current. But I am mystified about why in a pickup, the coil and magnet are both static where in a dynamic microphone these elements move in relation to each other.
Many thanks
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Trying to answer my own question:
Does the answer lie in the relative reluctance of air and metal? Does the static presence of the magnet make the oscillating position of a metal string relative to the air around it produce an oscillating current in the static coil by the string effectively producing varying reluctance properties in space? This is harder to get to grips with than when thinking about the way a microphone works because a moving magnet will produce a current in a coil as "lines of flux" cross the wire in the coil. If this is right then how could these lines of flux operate in a pickup if the coil and the magnet do not move relative to eachother? Can they be drawn or represented by an graphic animation?
Am I barking up the right tree?
Many thanks
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Yes - it's a variable reluctance device. As the string vibrates, the reluctance of the magnetic circuit changes which changes the flux through the pickup coil. This generates a tiny emf, which goes to the amplifier because the induced emf is proportional to the rate of change of flux.
This would suggest that the signal is, actually, the time differential of the motion of the string so you would need a lot of bass boost - 6dB per octave, even.
This question sounds like another homework assignment. ???
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Q: What is the magnet in an electric guitar pickup for? Why is it needed? Is it there to magnetize the strings? Couldn't magnetic strings be used instead? ... But I am mystified about why in a pickup, the coil and magnet are both static where in a dynamic microphone these elements move in relation to each other.
Usually you need to move a magnetic field (or change the field strength) in the vicinity of a coil in order to induce a current. As you say, in a conventional "dynamic" microphone you move the two relative to each other.
It's probably not practical to make a guitar string out of an alloy which has good mechanical/acoustic properties and as well as capable of acting as a permanent magnet. You could probably only magnetise it along it's length, and the strength across a short section (the pickup) would be very small.
I've never actually seen a guitar pickup close-up, but I assume the permanent magnet induces magnetisation across the (nickel-based?) wire so it becomes a temporary magnet... which vibrates enough when plucked to induce the required signal in the coil.
See also: http://www.igdb.co.uk/pages/beginners/all_about_pickups.htm
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it becomes a temporary magnet... which vibrates enough when plucked to induce the required signal in the coil
That's a way of looking at it but what you have done is to change the reluctance in the magnetic circuit - which has changed the flux through the coil etc. . . The strings are steel. You don't need a permanent magnet. It needs to be steel so it can be stretched. Iron would probably work better in terms of induced emf.
They have a really pathetic signal output, which is why electric guitars are (were) plagued by hummmmm.
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Hello. Thanks for answering folks.
I’m trying to get to the bottom of the how the basic properties involved interact for: a dynamic microphone, a loudspeaker and an electric guitar pickup.
Would you be able to confirm, clarify or correct any of the following:
A. In a microphone the movement of a magnet relative to a coil induces a current in the coil because it’s lines of flux are made to cross the coil plane.
B. In a loudspeaker a magnet is moved by the force that occurs when an electrical current crosses it’s lines of flux
C. Effectively in A and B the reluctance is a fixed value.
D1.
In an electic guitar, oscillating strings change the properties in the free space near a magnet modulating the strength of its lines of flux. This creates a modulating potential difference at the ends of the pickup coil.
or:
D2.
In an electic guitar, oscillating strings change the properties in the free space near a magnet, modulating the direction of its lines of flux. This creates an alternating current in the pickup coil.
Thanks very much
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When public telephones used large handsets the receivers used metal diaphragms and the magnet assemblies were regularly stolen to make guitar conversions
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D1 is the nearer of the Ds but you should use the term induced EMF.
ABC are right.
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Those old earphones made a functioning microphone - you had to shout~! The vibrating steel diaphragm performed the same function as the string.
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When Strowger ruled the telephone communications if you acquired enough skill you could tap out the number on the handset rest and get a free call.
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You certainly could. You didn't need to tap the 9s, because that had to work for 999. I got quite proficient at it.
I remember a trip round an exchange when the guy told us you could hear the change in rythm of the gear when tapped. He implied that the flying squad would rush round and nab you, Not sure if they ever did.
You must also remember the old 'tandem dialing' trick where you could go in and out of a local exchange to get to one that would have cost you a trunk call. You just had to know the local dialling codes for that exchange. I could ring from Leatherhead to Reading in several hops when the lines were good enough.
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We skilled old telephone hackers are a dying race now I can talk to my son in Australia for free via the internet
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Free????
If only it were free. No sooner have I bought a computer than I find the capacity is too low and I need more and more.