Peak Ratings and Speakers…Part 2

I realised after finishing part 1, that I’d missed my target by so much I could have shot myself in the foot. Nothing new there then.

What I was really after was some kind of sane explanation regarding how PEAK rating (or MUSIC rating; same difference) is the only one that makes sense in the context of speaker ratings. Instead of this, disappointingly, I was wont to ramble on about spectacles with springs in, and I think camels came into it somewhere. So I am definitely going to be disciplined and not have anything to do with hedgehogs, emu’s or camels. Now where did I put my specs with the springs in……

The problem with rating a speaker cabinet in RMS values, is that RMS, strictly speaking, deals with only one frequency. So an amp, preamp, output level of some ficticious device (we’ll call a toadophone) will be rated in RMS at one particular frequency. Often this frequency is either 1 kHz or 400 Hz. A second or two’s thought will be sufficient to prove that an RMS rating on a speaker cabinet tells you nothing at all that’s useful. This is a happy situation, because a second or two’s thought is all I’m motivated to string together on Tuesdays. Even though it is Wednesday.

What’s wrong with RMS then? Nothing, except that your bass speaker has to take maybe 4 octaves whereas your tweeter has only got to survive 1 octave. If this sounds unfamiliar, then I’m afraid you have to go back to part 1 and suffer camels and spring-loaded specs. Or was it spring-loaded camels? Well, anyway, if we think in terms of PEAK or MUSIC power this problem can be sorted out.

Let’s say we’ve got an amp that kicks out say 50 watts RMS. MUSIC or PEAK rating (for the same thing) works out to be double that (give or take). We saw that in part 1. So your speaker cabinet has to be able to handle 100 watts MUSIC. This means that all the speakers in your cabinet have to at least equal the 100 watts MUSIC output of your amplifier. If they were all exactly the same speakers, each one would take a third of the total 100 watts. So, 33 watts each (ish). A bass player’s 4 x 12 cabinet works exactly like this. 25 watts each for a hundred watt cab.

Taking a look at the picture we used in the last post (sounds like I should be playing  a trumpet. No, don’t go there) the three speakers are obviously not the same. This is where the power per octave comes into it. As we said, there’s a thing called a CROSS-OVER in the cabinet, and its job is to sort out which frequencies go to where. These are guesses, but not too far off. The bass speaker gets 4 octaves, the mid-range gets 3 octaves, and the tweeter gets 1 octave.

If we now say that the cabinet in total has to handle 8 octaves then each octave has an eighth of the total, so about 12 watts (just over).

From that, we can see that the bass speaker has to handle 4 of those total octaves, so 4x 12; is around 50 watts.

The Mid range, 3 octaves. So 3×12 or 36 watts. And the tweeter one octave, so 12 watts. 

If we want to go to the trouble of dividing those figures by two, we get RMS ratings for each speaker of ; 25 watts bass;

18 watts mid; and 6watts tweeter. Or thereabouts.

Here it is that we get to the point. If we rate the cabinet in RMS we have to state the frequency of the rating. So the cabinet is 50watts at 400 Hz, or 18 watts at 1kHz, or 6 watts at 10 kHz.

This is obviously completely meaningless, and worse than that, misleading. The power rating in PEAK or MUSIC power, however, makes perfect sense. It’s a 100 watts; and that 100 watts is sorted out within the cabinet by the cross-over.

I would like to have put a few more pictures in but I couldn’t think what for. Maybe a picture of my dog then? But I don’t have a dog. “Oww! I thought you said your dog don’t bite!”  “That’s not my dog.”  

Tea time. 

SJB Ant Reverb Head

This one might be a bit scrappy. It’s not even about the Ant model, but it might be quite instructive. Also, it could have used some pics, but it’s gone now so I’m stuffed on that issue. Apologies.

SJB, as I understand from no more reliable a source than hearsay, were made by a chap called Brian in Cornwall, UK, (Frome I think) who put together maybe a couple of hundred of amps under the SJB label.

Make no mistake, these might have been knocked together in Brian’s shed, but very nicely designed, a little bit quirky, and unusual in a few ways. Also hand wired, mostly point to point, with a couple of small tagstrips in there.

For me to put stuff on here, it has to be in some way interesting, and this was interesting not just because it was a very nice amp, but also because it had blown the mains transformer. The customer, needless to say, was unimpressed when I called it ‘interesting’. The real interest was that I had no information of any kind, at that time (you can turn up a schematic on the FIS site, which is brilliant) and I’d never seen one in my life before. Good start, not.

I had to guess the characteristics of the transformer so that my client could sort out one for himself. His idea, not mine. Where to start ? These are the broad questions you have to ask, because whoever is going to supply a transformer, is going to ask them of you.

What is the primary voltage rating. ( What voltage comes in from the wall socket?)

Does it have a valve rectifer?

What arrangement is the secondary HT supply? (Is it one winding or does it have a centre tap?)

What current and voltage is the heater supply?

Are there any other windings?

I’ll probably think of a bagful of other things when I’ve finished. Anyway, off we jolly well go.

————————————————————————————————————————————–

A valve rectifer has to have a heater supply. If the valve number starts with an ‘E’ then the supply is 6.3 volts and a guess at the current would be around 300 milliamps. Something like an EZ 81 you might find. The current is important because we have to add up all the heater currents to get the current that the transformer needs to supply in total for the heaters. However, this particular amp had a GZ 34 rectifier which is a 5volt supply, and therefore has a completely seperate winding on the transformer. It’ll be about 0.5 amps rated.

The next bit (the arrangement for the HT supply) will often be solved by the rectifier valve. If it’s a GZ 34 (or 5Y3) it’s almost always centre tapped, which means that there are three wires coming out of the transformer, two of  which would have been connected to the rectifier valve and the other to ground. The voltage is easy. Look at the main smoothing caps, which will have a voltage rating printed on them. Divide that by .707 and you’ve got something close to the overall voltage rating of the HT winding. The current isn’t so easy. It depends on the valve complement  and mostly what sort of output power you expect. This one had two 6v6′s and you would expect maybe 15 watts max out of those in class B-ish sort of biasing. If we divide that power by the HT voltage we’re expecting (this worked out to around 350 volts) it comes out to about 50 mA (milliamps). But we’ve also got to allow for the preamp valves, and that’s a total guess because there’s a lot of stuff affects that. If we say about 10 mA for each valve it’s probably not far out. This one had three preamp valves,  so 30 mA  and then a bit on top to be on the safe side, and we finish up with 100mA.

On to the heater supply. Nearly always 6.3 volts. The valves will be mostly E numbers (ECC 83; EF 83; etc. 12AX7; 12AT7 are USA equivalents but have the same heater volts ). The current is easy. About 0.3 amps per valve. However many pre amp valves you have, add them up and multiply by 0.3. Three valves will give you about 1 amp. The output valves are different. EL 84′s (the sort used in old Vox amps) take 0.75 amps each. 6L6′s take about an amp each. 6V6′s about 0.75 amps each, and EL34′s about 1.5 amps. You have to add these currents seperately and then add the total to the preamp current you worked out earlier. As a very loose guess the heaters on the output stage of a 30 watt amp might take 4-5 amps, on a 50, maybe not much more; on a 100 about say 8 amps.

From that lot we’ve guessed the entire characteristics of the replacement transformer, without having one stitch of information on the transformer itself. Well, I thought it was interesting, anyway.

Peak Ratings and Loudspeakers/Speaker Cabinets

There really is a wonderful amount of bullshit cast around on this subject. I could probably stand in the way of it and become a statue the size of the Albert Hall in less time than it takes to say “SPL”. Not bad for somebody not much over five feet tall.

Here are a few questions that would be handy to get answers for, in the interests of usefully composting said bullshit.

                                         1 Why can I pay (should I want to) twenty quid for this speaker, and twenty thousand for that one, when they look about the same?

                                         2 Will they sound any different? (Assuming I’m not as deaf as an Emu. No offence to Emu’s intended).

                                        3 Do I really need a number of different speakers in the same box?

                                        4 Why does the little tweeter speaker make smoke signals if I shove bass into it? Is it trying to tell me something?

“Can of worms” is a phrase that immediately springs to mind. But, when you gotta go, you gotta go; so here we go.

This a speaker cabinet. This might seem as though I have dropped suddenly into the advertising business. It would appear to be a statement to which even a moderately intelligent hedgehog might take exception. But if we’re going to start at all, it may as well be from the beginning. A speaker cabinet is a box that you put speakers in. So far so good. But even that, seemingly simple sentence, is fraught with problems.  I feel a multiple tea break job coming on.

So, just a look at the pic will tell us that its definitely a box, and that it’s got three round looking things in it. The one at the bottom is the ‘woofer’ or ’bass speaker’ (they’re called ‘drivers’ these days but I think of them as being the opposite of ‘conductors’. Anybody not born in Blackpool before 1950 won’t understand this.)

The one in the middle is called the ‘mid-range’ and the one at the top is called ‘the tweeter’ or the ‘high frequency transducer’ or some such. They each do different jobs. This is where we need to start thinking about sound as a kind of spectrum. In light terms, red is at the low end of the frequency spectrum and violet at the high end; and if we take it a bit further, infra-red is a bit lower than red and ultra-violet a bit higher than violet. Thought of in those terms, the bass speaker would be at the red end of things and the tweeter at the violet end, with the mid range filling in the bit in between. They’re all obviously different looking things, but why? Without going into stuff about wavelengths of sound, the common sense answer is that bass frequencies need mass to shift the air, and this comes down to something called ‘power per octave’. This might make more sense after a bit more common sense background. And then again, it might not.

We need to know what an OCTAVE is for  a start. In music it’s eight notes. CDEFGABC on, say, a piano is an octave. So C up to the next C. Eight notes. Or G up to the next G. You get the idea. However, if you tried to explain that to any speaker cabinet I’ve ever met, you would not get much sense out of it.  One octave of Sebelius to a speaker cabinet would be little different to one octave of the Stranglers, but I can’t imagine Sebelius doing backing tracks for the Stranglers. Don’t know where that came from.  The point, if there is one, is that a speaker cabinet deals with frequencies; and the notes (so far as it is concerned) is just a byproduct of those frequencies. So what? So we need to know what an octave is in terms of what a speaker understands; which are frequencies.

This is simple. For a frequency of say 100 hz (that’s 100 vibrations per second) an octave higher is 200 hz; so double. An octave lower would be half, so 50 hz. This is where it gets interesting. Honest.

If the lowest frequency that the cabinet can take is say 50 hz, the next four octaves are compressed into 750 hz. But between 10,000 hz and 20,000 hz (10,000 hz altogether) is only one octave. That’s all that the tweeter has to handle. The mid range, might have to handle three and a half octaves. So as the frequencies get higher, the power handling requirements grow less. A lot less.

I would like to go into the details of why speakers look like they do, and how one works; but that would fit better elsewhere. It’s worth mentioning this, however. If the wrong frequencies are allowed to go to the wrong speakers, a variety of things are likely to happen. If high frequencies go to a bass speaker, it won’t do anything. No; it will do something, but it won’t do anything useful. The bit that drives the cone that you see from the front, is just a coil of wire on a tube (often cardboard), that moves in a magnet. It slides in and out of a gap in the magnet called a SPEECH COIL GAP. The idea is that it pushes the cone back and forth, thereby moving the air in front of it, but the cone itself has a certain mass and that will only allow it to move so fast. What happens when the frequency put into it becomes higher than that to which cone can respond, it just accepts the power and heats up. It has to convert the power into something, because it won’t just go away, and if it can’t convert it to movement (back and forth) it has to become a small electric fire, and convert it to heat.

The wrong frequencies are equally bad for the other speakers, but for different reasons. Because the power per octave is much greater with bass frequencies, a tweeter is driven towards doing the hundred metre sprint out of your nice cabinet. I’ve seen one or two with the speech coil hanging out like those strange spectacles with the eyeballs on springs.  The next question is, “How do we get the frequencies to go to the right speakers, because I have a nice open fire in my lounge and I don’t really want another in my speaker cabinet.” Quite right, too.

As we’ve got the general idea of  what does what and more or less why, I should now get a cup of tea, and leave cross-overs to part 2. Cross-overs, you’ll be disappointed to know, have nothing to do with the transvestite fraternity. So far as I know.

Peak Ratings

I’ve looked forward to doing this one as I would getting my head nailed to a fence. Not much.

For a start it’s nothing to do with T.V., Coronation Street, or traffic jams. That was easy. I’m pretty good at what it’s not, then. Rating anything at PEAK should be as easy as a handbrake turn on an Audi Motor sport four-wheel-drive, but it’s actually as simple as the same thing in a Robin Reliant; and if you’ve ever tried that, you live dangerously, and I take my hat off to you. Anyway, here’s something that might convince you that I’ve haven’t left for a warmer place. Chance would be a fine thing.

My inaccurate version of a sine wave, we’ve seen before. The square bit we haven’t, and this is about where I wish for a lifeboat. This is all about PEAK RATING of, say, an amplifier. To deal with PEAK POWER rating is slightly more complex, so we’re dealing with voltage ratings at the moment. The idea is the same, though.  You may have noticed that PEAK rating is always the same as MUSIC rating. In order to notice this sort of thing (as I obviously do) you have to be in possession of a yellow and pink striped anorak with ‘I love peak ratings’ stamped all over it.  I digress.

Why is MUSIC like PEAK then? PEAK tells you how much (power, voltage, current  etc,) your amplifier can put out. That’s it. The amp can’t deal with any more for all sorts of reasons; reasons that, if you have the right anorak, you can find out about in various other articles in this blog.  Music, is hardly ever a sine wave. A sine wave is the most singularly boring thing to listen to for more than a few seconds at a time. Apart from the odd three note bass line that sounds quite promising and then an hour later is still there, accompanying somebody trying to eat the settee they’re sitting on. Matter of opinion, of course.

Here, we start getting into what a musical tone is, because this is all wrapped up in PEAK rating. The first thing to notice about the drawing above (this, I’m going to credit to my cat; it’s that bad) Is that there are three separate wave shapes. There’s the original sine wave which is easiest to trace from the peak at B. There’s a little wobbly one labelled the ‘Third Harmonic Wave’  (this is the one that looks not unlike several copulating camels. Not that I’ve ever seen that sight.) Then there’s the VERY WOBBLY ‘resultant wave shape’. If we compared that one with the square wave we had earlier, it’s possible to see this wave shape as an approximation of the square wave. This would be helped no end by a few stiff scotches, at which point it wouldn’t any more similar, but you really wouldn’t care if it walked into your lounge with the camels, and the lot joined you in eating the settee. 

The RESULTANT WAVE happens when we combine the FUNDAMENTAL (the original sine wave) with the THIRD HARMONIC, this last one being three times the frequency of the fundamental. Or in other words, we get three complete waves of the third harmonic for every one wave of the fundamental. I have someone I can legitimately blame for this. A French guy called Fourier came up with it, along with a lot of other clever stuff. The idea is that, whatever the wave shape (whether it be the result of a shipwreck, flatulating camel, or bad baritone; none of them all that disimilar I am assured) it can be simulated by the addition (or superimposition, if you’re happy with words that never end) of various harmonics of various frequencies and various amplitudes.

Every one of these harmonics has its own value (of power, voltage or whatever) and therefore adds this value to the original sine wave. This is why the original fundamental has the least value, because any alteration to it, adds value.

A  square wave carries the maximum value of power from an available peak. It could be thought of as having no gaps in the waveform. It’s as full as it’s going to get. It can also be constructed, in the same way as any other shape, as we said earlier, from harmonics. In order to produce a square wave we have to use all the odd harmonics (3rd, 5th, 7th, 9th etc) to infinity. Obviously not possible.

The big problem in producing a perfect square wave, is it’s LEADING and TRAILING EDGES. A perfect square wave gets from its highest (positive) value to its lowest (negative), in NO TIME. Or, if you like, the high value’s finish point is the same as the low value’s start point. They exist in TWO PLACES AT THE SAME TIME. This I find a very worrying concept. Oh well, I can aways get a bag of chips and forget about it.

Music at it’s most complex could be thought to be a lot of harmonics. I doubt if Rachmaninof would have got much in the way of inspiration from that thought. But this is what PEAK/MUSIC rating is all about. If we take it that a square wave carries the most power and it has the same peak value as the peak of the sine wave, then theoretically, that is the most the your amplifer is going to have to handle.

As a parting shot, PEAK or MUSIC power rating is twice the value of RMS. So the same amp will be 10 watts rms, or 20 watts peak.

But the real useful part about Peak or Music rating is in its application so far as speakers/speakers cabinets ratings are concerned. Why? Because the function of a speaker is to reproduce MUSIC. which is not one sine wave frequency, but a lot all mixed up. This is where Music Power really starts to make sense. “It’s about time” did I hear you say? It’s certainly TEA TIME……..     

 

R.M.S. part 2

Well here it is, part 2; if you can stand to embark on more of the same sort of trek as Part 1.

Here’s a nicely inaccurate drawing (again) of a sine wave. You’ll need to keep referring back to the drawing of the generator in the RMS part 1 blog, for it to make any sense.

  The vertical line (called the vertical or Y axis) tells us how much of whatever it is we have, we have. So to speak. In this case it’s volts, the top of the line (axis) being positive, and as we move downwards, we go through zero (where the ‘X’ or horizontal axis is positioned) and on down to increasing negative volts. The path of the sine wave says what happens in the coil (of blog ’RMS part 1′).

If we start at the left, the badly drawn sine wave is at zero, and this happens when the magnet is at position ‘b’ of its rotation. We have to remember that the emf (electricity) in the coil is only produced when the magnet flux (which is a posh name for its magnetism) crosses (or cuts) the windings of the coil. When it’s at position b, it is for a very short time, travelling in line with the coil, and so produces no emf in the coil at all. So, the zero volt on the sine wave. If we travel 90 degrees further round with the magnet, we get to the point of maximum emf produced by the interaction of the magnet and the coil. There is nothing clever about this; it’s just that the magnet is closest to the coil and moving past it at its maximum speed, so it produces more of the emf we were talking about. This interaction between magnet and coil is called INDUCTANCE and has produced the PEAK of the sine wave (also called the ANTINODE), its highest voltage. As the magnet continues round its circular path, it moves away from the coil in exactly the same way as it approached it, until it reaches position b on the opposite side of the rotation. 

All this completes the POSITIVE HALF-CYCLE. All we have to do, to figure out where the next bit comes from, is to remember that it’s the south pole of the magnet that is doing the same thing as did the north pole, and so produces the opposite effect and goes negative. This completes ONE CYCLE, at which point the whole procedure is ready to start again. And if you think I’m tedious, you should consider how bored that magnet is.

So everything has gone through a complete revolution. The problem we’ve got when it comes to figuring how much of anything there is of anything, it’s that it is changing all the time, 50 times per second  (60 in USA). We can’t average it, because the power generated goes exactly the same value positive as negative, so the average is zero.

Now any of us who might have inadvertantly grabbed hold of a couple of wires connected to the mains supply, knows for a fact that zero volts it is NOT. 

To call it a PEAK value is fairly obvious, because the sine wave reaches the same peak in either direction and, so long as the coil, magnet, and rotation remain the same, so does the PEAK voltage. But it only happens very briefly, so doesn’t really tell us what we want to know; and that is what kind of usable emf stuff we have available.

And here we are at…………..wait for it………..R.M.S. !!!!!!!!!   It stands for Root-Mean- Squared or, more understandably (sort of) the square root of the mean, squared. So how do we get that?

First we get the mean squared (which ’means’ the average of the sine wave). We do this like this: we take a measurement at a point on the wave; then another, then another etc. The more we take of these the more accurate is the final answer. Then we multiply each measurement by itself (we ‘square it’) and add them all together. To get the average (Mean) we divide this result by however many measurements we made.  

I’ve done this just three times on the diagram below.  0.6 squared is 0.36 and 1 squared is 1. If we did the same thing on the negative half of the cycle we’d finish up with 6 sample points, and the good bit about the squaring thing, is that the negative becomes positive, because that’s what happens if you square a minus thing. So now we’ve got six points on our wave to average out (produce the mean square) They all add up to 3.44 and if we divide that by the number of sample points, which is six, we get 0.57. So far so good?  We’ve now got the ‘mean square ’ bit. To get the ‘root’ bit, we take the square root of the ‘mean square’ (which was 0.57) and that works out to be 0.755. Which is wrong. Whooops. It’s wrong because we only sampled the wave in six places. Even if we sampled it in a 1006 places it would still be a bit out. The R.M.S. value of a pure sine wave is always 0.707 of the peak value, which is a handy number to remember. And working the other way round, the Peak value is 1.414 times the R.M.S.  Just to put meaningful spin on this. U.K. mains voltage is 240 volts R.M.S. But the PEAK voltage is actually 1.414 times that; which is nearly 340 volts. Same thing, different number.

Just as a parting shot; ANY wave shape can have an R.M.S. value; but the sine wave carries the least power. And this is where the PEAK rating can be useful, particularly in speaker ratings.

Watch this space, (if you have trouble sleeping). Teeeeeaaaa Time!

R.M.S. (Reminding My Self?)……..

This is not really about anything, much.

I came across an article on rms power, and was reminded why it was that I do this blog at all. It doesn’t attract a flood of business. I probably wouldn’t know what to do with it if it did. It’s certainly not an ego-massaging exercise; most of the time I consider myself to be quite daft. So it doesn’t make me look good, and it doesn’t make money. What other motivation is there (other than blatent stupidity, of course.)?

Well, I don’t like to see decent, intelligent folk, being taken for a ride by folks who aren’t very decent, nor necessarily very intelligent. They just happen to have a lot of luck, money, or both. But maybe not much else. What is this bloke talking about? You might well ask.  For the sake of a little snippet of information, here and there, these decent, and intelligent folk (which is most of us, don’t you think?) can be placed in a much better position to ask a few awkward questions of the afore-mentioned sharks, put them in the odd embarrassing position, and save ourselves a few quid into the bargain. That’s about it. But specifically speaking this is about r.m.s. Not normally used in the context of  ‘reminding myself’.

RMS, PEAK, MUSIC, PMPO, are all familiar to anybody who has tried to make some sense out of buying a power amp. RMS, PEAK, and MUSIC are all straightforward engineering standards. PMPO is a piece of subterfuge. We aught to sort that out first.

Peak Music Power Output. This would indicate the amplifier’s capability of dealing with a transient peak (like, say a cymbal or tambourine or some such). This would be present for a matter of milliseconds. Were it possible to sustain this kind of output, your three million watt car stereo beat box would probably turn your car into a heap of smouldering metal. But it can’t do that. The way it can produce that sort of power so briefly lies within the power supply which will in some way be highly capacitive. All this means is that the smoothing capacitors within the power supply will produce  a momentary surge to handle the transient (cymbal or what have you) and then it will have discharged to its rms level output, of not very much. It also carries some very unpleasant compression characteristics; pumping and the like.

So its a piece of deception. The more cynical amongst us might say that there’s not much marketing doesn’t fall in that category. You could jot the manufacturer’s name, and maybe where you bought it from, in your diary. If they can try it on for that, they probably won’t be too troubled by conscience if the opportunity arises again in another context.

RMS, on the other hand, is real. It means Root-Mean-Squared. This isn’t all that easy too explain (but I’ll have a go), but the reason for it is. If you apply a 10 volt (say) rms signal to a resistor, it will draw this much current (whatever that is). If you put a 10 volt battery to the same resistor (that, then, will supply d.c.) it will draw exactly the same current. And that’s all it is. Rms voltage/current/ power has the same supply potential as the same value of d.c.

Is this significant? And why bother? Well it makes a lot of calculations that much easier. A 20 watt output from your amp would need to put out exactly the same voltage into the load (speaker or what have you) as battery supplying twenty watts would. It’s not quite like that because an inconvenient term called ‘the reactive component’ which sounds like it should be dangerous but isn’t. Not compared with human reactive components, anyway. 

To figure out what any quantifying convention means, it’s probably a good idea to know what it is we’re going to quantify.  There would seem to be little point in quantifying, say, a camel, if we thought it looked like a seagull. What do I mean? Well if I were in the middle of a desert I know what I’d prefer a 100% of; and it wouldn’t be the seagull.

So probably best to sort out what it actually is, or at least where it comes from,  before trying to sort out how much of it we’ve got. If you get my drift.

 

This is an ac (alternating current) generator in its absolutely most basic form; but you’d have to say that an actual generator (although there are different variations on the same thing) is just the above diagram with a lot of the things marked ‘coil’ in it, and  a lot of the things marked ‘magnet’ in it. This is what happens.

One thing we need to know about first of all is this. If you waft a magnet about in front of a coil of wire you will generate something called an emf. This stands for ELECTRO-MOTIVE-FORCE, and that means that usable electricity will exist at the two ends of the coil that we’ve marked as ’OUTPUT EMF’. The output emf can be measured as a voltage, but it’s actually more than that, it’s a potential source of power. This depends on the magnet moving; if it doesn’t move the coil of wire is simply that. A coil of wire; and therefore (unless you happen to have some sort of  ‘coil of wire fetish’ ) boring.

I should make an apology here. If I were clever with animation I would have the magnet rotating about a centre axis. But the last animation I tried took me a fortnight and I finished up with ten seconds of a stickman walking with irritable bowel syndrome. So you have to use your imagination.

This magnet would rotate at 50 revolutions per second to produce 50Hz ac (UK) or 60 revs per second in the USA. The magnet, passing across the face of the coil generates  a voltage at the ends of the coil of a particular polarity (That is, one end of the coil is either positive or negative with respect to the other) and this depends on the polarity of the magnet and way the coil is wound. If it is wound, say anticlockwise, the output would be of opposite polarity to if it were wound clockwise. Similarly for the north-south orientation of the magnet. At ‘a’ the magnet is travelling at its fastest across the face of the coil. It is also closest, and for these two reasons the ‘LINES OF FLUX’ of the magnet sweep across (‘cut’) at their highest density and fastest rate than at any other point on the magnet’s travel, which produces the PEAK output at the coil ends. As it moves around the path of rotation this effect becomes less, until at ‘b’, the magnet is actually travelling in the same orientation as the coil and so isn’t sweeping across the coil face at all. So from ’b’ to ‘b’ passing ‘a’ is one half cycle, let’s say the positive half. But now the opposite pole is tracing the same path past the coil, and the only difference is that the opposite pole will produce the opposite voltage in the coil.

Below is a very poor drawing of a sine wave. If you’d like to look at some good ones go to Google Images and put in Sine wave. If it were a camel it would still look a bit like one. Well, alright, it wouldn’t look like a seagull. Back to the camel/seagull thing. I worry about myself sometimes.

 

I would hazard a guess that, even if you’d spent the last twenty years floating about on a raft in the middle of the Pacific, you would still recogise the above diagram. The sad bit is that, although its as familiar as your own toilet, nobody ever seems to explain what it means.

I’m going to have a go at that, but my thirst for tea has assumed monumental proportions, so I’ll do a ‘Part 2′ of this and carry on from where we left off.