Elephone wrote:Any thoughts on whether this is likely to be damaging to tape?
Well, yes... damaging to the tape and to the machine you later try and record the tape on... :shock:
First, the physical surface of the tape is likely to be damaged by anything it has to scrape over, which is why tape guides are often designed to rotate, and are always highly polished to minimise friction. Edge damage to the tape is also a concern if the tape edges rub against the top or bottom of the guide mounting.
It's true that that basic arrangement -- using a very strong compact magnet moving along the length of the tape -- will probably cause some erasure of any pre-recorded material on the tape... Any strong magnetic field passing through the tape will re-align the magnetic particles in the tape and thus damage the original recording through some degree of erasure.
But how effective that specific technique will be depends on the field strength created by the permanent magnet and the recorded fluxivity on the tape. And it may well take several passes to achieve a satisfactorily complete erasure.
However, the magnetic field left on the tape from the process as described will leave all the magnetic domains facing more or less the same way, and will effectively create a strong DC-bias effect on the tape that will likely cause magnetisation of the tape machine's transport and heads when the tape is re-used. That single-direction field may even overwhelm the machine's head's ability to recording anything new at all... particularly if it's an old recorder working with relatively low recorded field strengths anyway. (Edited: And as Mr G says below, it can also leave clicks on drop-ins and edits!)
Moreover, the idea of willingly placing a strong magnet anywhere near a tape transport in the way illustrated is just wrecklessly stoopid in my view... :o
There's a very good reason that professionals use AC-erasers -- devices with a strong electromagnet running off an AC current -- which is that it leaves the magnetic domains in the tape randomised, not all facing the same way... so there's no risk to magnetising anything!
I've seen plenty of mains-powered bulk-erasers -- like the one in your second link video -- on ebay and various specialist tape machine website before now. If you really feel you need to bulk-erase tapes that's the way I'd do it... and I'd keep the eraser well away from any tape machines (and wind-up watches) too!
With the type show, you place the spindle of the reel on that spigot near one end of the box, and then manually rotate the rape reel a few times so that it is all fully exposed to -- and passes through -- the magnetic field generated by the electro-magnet at the other end of the box. In that way the magnetic domains in the tape see the magnetic field build to a maximum and then decline again. It's normal to turn the tape reel over and repeat the process to ensure full erasure.
But I guess the important question to ask is why do you feel the need to bulk-erase quarter-inch tapes? On most machines the erase head gaps are intentionally wider than the record/play gaps to ensure full erasure anyway. It requires a real-time pass rather than a few passes over the AC-eraser... but it's a lot cheaper and probably better for the tape!
edited -- Folderol got there before me! And he's absolutely right... please don't do the magnet thing!