I finally had a chance to check the strength of some magnets against some breakaway magnets we had laying in the shop. The ones I tested were the old 60's magnets. I didn't test the later style yet. I left my notes in the shop so I don't remember the exact readings. I'll post them later if I can remember to.
A little preliminary info:
The strength of most of the Alnico V magnets I have tested in the past are in the range of 750-950G. Depending on the composition of the alloy, and whether the magnets are saturated, the readings can be lower or higher. But those readings are typical, generally speaking. Alnico II typically checks a little lower, in the range of 550-650G. Again, depending on the aforementioned factors, the readings can be higher or lower. Alnico IV is similar to Alnico V in terms of field strength. Maybe a little stronger, on average. But again, variations in the alloy mix can cause higher or lower readings.
Ok, onto the breakaway magnets...
When I tested the breakaways, all of their readings were relatively low and the poles were mismatched. One particular magnet read in the low 200's on the North pole and read in the low 400's on the South pole. I remember this particular magnet. It came out of a set of pickups from a customer who complained that his pickups had lost volume and clarity in recent years, sounding muddier and muddier with very little volume compared to when they were new. I chose this as my primary test subject due to it's peculiarity.
I decided to take the magnet mentioned above, an Alnico II, and an Alnico V that have been laying in the shop for almost two years. My plan was to saturate all three and compare the readings. As I mentioned earlier, I will have to look at my notes out in the shop for more precise readings, but after charging, the Alnico II was reading on the low side of 700G (about 730, if my memory serves me correctly); the Alnico V was reading just under 950G; and the 60's breakaway magnet was reading around 870G. I don't remember exactly, but the poles were pretty even after charging.
Based upon those readings, I would have to say that it seems very reasonable that the 60's breakaway magnets are likely either Alnico IV or Alnico V. Alnico IV generally has a more transparent tone, with a flat frequency response. (I'm working on a device to test that soon.) Depending on who you ask, Alnico V usually pushes the mids, has more high-end, and "tighter bass response." Personally, that translates into garbage to my ears because if we have more bass, more mids, and more treble, then the frequency spectrum is still flat, it really depends on exactly which frequencies are accentuated or attenuated. I think Alnico V has less highs and more mids. But it depends on who you ask. Beauty is in the ear of the beholder. There are other factors, such as whether the magnets are anisotropic or isotropic. I believe the breakaway magnets are isotropic, whereas the Alnico magnets I tested are, without a doubt, anisotropic. According to some people, an isotropic Alnico V usually reads a little lower than an anisotropic Alnico V and has a little "warmer" tone.
Now, as for the ridges...when I put the test probe on the side or surface of the magnet I get a normal reading. When I put it between the ridges, it drops a little. But when I coupled it with a coil and a row of pole-pieces the readings were EXACTLY the same from pole to pole and in-between the poles (exactly 623G, as I remember.) The readings drop dramatically when I moved the probe to about 1/8" away from the side of the screw head. In fact, the drop was around half of what I was getting at the poles. The further I moved the probe from the poles, the greater the drop.
I should mention that a general rule of thumb is that the stronger a pickup magnet is, the more output and the higher the resonant frequency. So if some old Mosrite pickups are sounding a little weak or muddy, but you're getting a reading from the lead wires, then don't be quick to dismiss them as a lost cause. The magnets may need to be charged. As a pickup maker and researcher, I can say that this is very enlightening. While these tests are not conclusive, we do now have some reasonable plausibility for believing that the breakaway magnets were Alnico V; but there is also room for doubt as to whether they could be Alnico IV. We also do not know whether they are anisotropic or isotropic magnets. I would wager they are probably isotropic when taking into consideration the manufacturer's intended usage and target audience. But that's just my opinion, it doesn't necessarily mean anything. Another variable that must be taken into consideration is the relative strength of the magnets. Were all the magnets saturated? We likely have no way of ever knowing. Again, taking into consideration the manufacturer's intended usage and market, they may have been given a nice zap back at the factory that would fully saturate them. But how long did they lay around before being installed in a pickup? They were probably stuck together in "bricks" from the manufacturer until being broken off and installed. When magnets experience friction with other metals and other magnets they experience "wrenching", which can degauss them and even manipulate the orientation. So what was the strength by the time they reached the end-user? That could be the spark to a never-ending debate. I would say that it was probably between 500-700G, but that is very debatable. Using my test magnet above as an example, I would say that since the pickup it was taken out of was North-oriented and the North pole of the magnet was reading in the 200's; and the customer complained that his pickups had weakened and become muddy over time, that is a pretty good indication that most Mosrite pickups were probably not that low to start with upon leaving the factory. That is my sole basis for proposing the 500-700G estimate.
The important thing to remember in all of this is that pickup tone is either directly or indirectly affected by many variables - machine vs. hand or, "scatterwinding"; gauge of wire; the size of the insulation on the wire (e.g. "red wire" vs. Formvar wire, or "green wire"...etc...); the size and shape of the coil, which is directly dependent on the bobbin; the bobbin material; the type, size, shape and permeability of the pole-pieces; and it just goes on and on and on. When it comes to Mosrite pickups, I feel that the choice of magnet is important when attempting to reproduce the original sound. To my knowledge, Ed Elliot and Curtis Novak both use Alnico V in their Mosrite reproductions. We use Alnico V as well. I would venture to say that I believe that we are all on the right track. I've heard great things about Ed's and Curtis' pickups and I hate comparisons between their pickups and ours. I think we are all reproducing very close and accurate reproductions. And while most everyone has been sure (not to mention satisfied) with the tone that we, Ed, and Curtis are getting from Alnico V magnets, I don't think any of us were 100% sure they were the "correct" magnets. WE are still not 100% sure, but I hope that my recent test may shed a little more light on the matter. I certainly think it is more plausible to believe that Alnico V are a good match in Mosrite reproduction pickups. I would like to begin testing Alnico IV eventually and compare the tone with the vintage magnets.
I tried to avoid begging the question as much as possible, but there are few things that 