From: S-T Morrissey sean@...........

Date: Thu, 13 Jan 2000 21:22:16 -0600 (CST)

Re counting turns when winding a coil: I don't bother to count, but I know the approximate number. First of all, I turn the coil by hand on a short shaft under good light, placing each turn EXACTLY adjacent to the previous, and delimiting each layer with correction tape strips as I described a few weeks ago. (need a repeat??). It takes about 2 minutes to make about 100 turns. And another 10 minutes to epoxy the layer. A 12-layer coil takes me over 2 hours, depending on distractions while the epoxy cures. This lets you know the number of turns by knowing the diameter of the wire and the winding length. I use #36 wire from Newark (need a repost of that info??; they take MasterCard!). "Beldsol" enameled #36 wire has a diameter of 0.0055", so if I have a winding length of 0.5" with EVERY TURN exactly placed, I have 91 turns in that layer. Since I don't use a bobbin form, I step back the end of each layer by one turn, or conversely start with a slightly larger number in the first layer. So I use a starting length of 0.615" ( slightly under 5/8") to start with 111 turns; subsequent layers are 109, 107, 105, 103, 101, 99, 97, 95, 93, 91, and 89 turns in the 12th layer, for a total of 1200 turns. The average length of a turn with a 5/8" initial diameter is 2.17", so there is 217 feet of wire in the coil, and at 414.8 ohms per 1000 ft, I should get 90 ohms, which is about what I measure. For your info: other wire diameters and resistances: (diameter for enamel insulation) NEWARK roll length, ft cost #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 Actually, the exact number of turns is not important. What is important is to get the maximum number of turns inside the gap of the magnet, which cannot be done with a "scramble wound" bobbin that often results from using a drill. The required bobbin alone will take up an appreciable amount of the volume. Also a target resistance value is not necessary. The trade-off in my opinion is how small a wire you want to work with to get the maximum number of turns into a given space. #40 and up wire is probably not amenable to precise layering, but will give more turns and more resistance. But remember that the output increases only with the square root of the resistance. 200 ohms will produce 1.4x the signal of 100 ohms. If I use the same dimensions as above and #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 about 4800 turns. With the same mean length per turn, this will be 868 feet of #42, which will measure 1440 ohms. So I get only 4 times the output (4 x 1200 turns) at 16 times the resistance (16 x 90 = 1440 ohms). BUT I also have 16 times the Johnson noise due to the resistance, and it may be difficult to damp with a resistor if the magnet is not strong enough. Regards, Sean-Thomas _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)

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