From: sean@...........

Date: Mon, 4 Sep 2000 13:39:49 -0500 (CDT)

Barry, I haven't made a formless coil for a speaker magnet because I haven't found a temporary form of the exact dimensions to wind it on. (see my write up on making the formless coil.... or do you need a repeat of it). But I haven't tried either, since I can make a reliable, reproducible rare earth magnet assembly with McMaster parts for $30, rather than dismanteling speakers. (also written and posted). But when you play with the formulas, the output is a function of the total winding length. This means that the output increases directly as the number of turns increases. However, to get more turns into the same winding volume (the cylinder of wire in the magnet gap), requires smaller wire, obviously. Wire of 1/2 the cross section area, like # 38 is half the area of #32 (from wire tables), will result in twice the number of turns so twice the wire length and output. But half the area is twice the resistance per length, so the winding resistance increases by a factor of 4. For example, increasing the turns conveniently by using wire of 1/2 the diameter with the same coil dimensions, if I use #42 enameled wire, which is about half the diameter of #36, I will get twice the turns per layer, and twice the number of layers, for 4 times the turns and output. With the same mean length per turn, this will be four times the wire length, but it will measure 16 times the resistance. So I get only 4 times the output at 16 times the resistance, BUT I also have 16 times the Johnson noise due to the resistance, and it may be difficult to damp (as a moving coil sensor) with a resistor if the magnet is not strong enough. size diameter resistance NEWARK roll length, ft cost #32 0.0088 164.1 #34 0.0069 260.9 #36 0.0055 414.8 ohms/1000 ft. #36E1321 1/2lb 6400 $27.33 #38 0.0044 659.6 #36F779 1 lb 19300 $73.49 #40 0.0034 1049.0 #42 0.0028 1659 #44 0.0023 2593 And another consideration for a coil used for a broadband feedback sensor, high coil resistance leads to instability. Anything much over 100 ohms is a problem. This became quickly evident when manufacturers tried to add feedback to existing seismometers with constants of 50 to 200 Newtons/Ampere (Volts/meter/second) but thousands of ohms. Increasing the coil magnet output is an advantage to a point for a simple moving coil sensor, but for feedback it does NOT increase the output: in fact it decreases it, where k=M/(G*C). However, it does improve the high frequency response of the feedback loop, so there are tradeoffs. Regards, Sean-Thomas __________________________________________________________ Public Seismic Network Mailing List (PSN-L)

Larry Cochrane <cochrane@..............>