How about a description of who and what the project is for? Of course, you could have mentioned it earlier, but I'm too lazy to go back and search (as in "I've fallen and I cannot get up!").

 

I measured resistance between the two wall-plug-side terminals on the transformer with nothing plugged in. I get 12.0 to 12.1 ohms.

 

Okay, here's the beast. It will be used to track the calibration drift (or hopefully lack thereof) of an EMDEX II EMF meter that I will be using for magnetic field monitoring for work. We decided we wanted a secondary standard to ensure that our device is working properly.

Note: the EMDEX II contains 3 coils to give multiple orientations so you don't need to struggle to orient it relative to the source. I have been able to achieve 3 constant measurements on 3 of my 4 coil resistances. I think I figured out what was interfering on the 4th one (resistors in the array were a little close to each other)

The coil needed to be portable and durable and as user safe as possible. I designed it with solid brass bits as much as feasible to minimize ferric influences. The control box is mounted in the lid so that it ends up as far away from the coil as feasible. In addition, the lid with the control box attached can be removed and placed approximately 3 feet away from the coil.

I haven't tested it like that yet, but I will. I need to clean up my floor first. I also need to make a bracket to keep the meter centered on its three axes. That will be interesting. Probably, it will consist of some resilient packing foam.

It is a little bulky, but not bad considering I built it all using only hand tools and an electric drill. Enjoy the photos. I hope you like it. After all, you all helped!

 

Here's the dial side of the control box.

Also, imagine all of this is varnished or something. It will be prettier some day.

 

I measured resistance between the two wall-plug-side terminals on the transformer with nothing plugged in. I get 12.0 to 12.1 ohms.

Well, I guess we'll see if the turn on surges pop the fuse or not. In retrospect it was a silly thing to ask you to do. A perfect transformer would have no DC resistance at all but of course, they don't exist.

 

Funny. My 4700 and 47 ohm resistor arrays and no resistors give nice, stable magnetic field outputs. 150 ohm resistor array fluctuates with an amplitude of around 2 milliGauss. Any thoughts as to why that would be? Should I look into a ferro (is that the right term) transformer or is this some kind of harmonic or something?

 

Here's the dial side of the control box.

Also, imagine all of this is varnished or something. It will be prettier some day.

I have to say that I was smiling while viewing your pix. Especially, the carpeted floor in your wood shop!

You did a fine job there and the cuts appear to be straight and square. Did you use a miter box? I think it looks great and your wiring looks neat and professional too!

Think Watco or Formbie's Danish oil. It maintains the warmth of the wood.

 

Funny. My 4700 and 47 ohm resistor arrays and no resistors give nice, stable magnetic field outputs. 150 ohm resistor array fluctuates with an amplitude of around 2 milliGauss. Any thoughts as to why that would be?

Are they getting hot? If you poke at them does the meter jump around. I came into this thread a bit late. Someone else (I believe) helped you out with calculating their values and wattage. I just assumed that part of the project was worked out. Check for a cold solder joint... that's what the poking was for.

Edit: Also check for dirty switch contacts. Rotaries are self cleaning but if they sit a long time they can get oxidized.

 

Glad you like it. The floor of my "shop" is actually the floor of my living room, which is now finely dusted with sawdust. Not the first time that's happened either. Japanese wood saws and miter boxes are my friends, but they aren't my carpet's friends.

I'll definitely check the resistor array. I'll use my thermometer thingie again. I may have fatigued the connections when I unsoldered things and put them back together again when I checked the switch for functionality.

So far, no fuse pops.

 

I should note, my solder job looked even nicer before I resoldered everything. It got a bit messy. Meh.

 

Nice work Maxine!

Only one comment - is this a true Helmholtz coil?

My understanding was that there are two windings with a geometry so arranged that one obtains a region of nearly constant (accurately known) flux density in a region between the two coils along their common central axes.

I can't see whether there are two parts to the winding or only a single coil.

 

Only one comment - is this a true Helmholtz coil?

My question too, t_n_k, after seeing the pictures. IIRC, the separation of the coils (either round or square) for Helmholtz coils is proportional to their radius. If you're interested, I have a copy of Grover's book on inductance calculations somewhere and I could look the stuff up for you. Or, it can probably be found on the web somewhere; I'm too lazy to look.

Nice job, Maxine -- done in a very workmanlike manner (or would that be workwomanlike manner? ). Reminds me of building a cradle out of particleboard in my apartment for a friend's wife's first baby when I was a student.

Oooh, I like the crosspiece -- is that Western maple or rock maple?

A couple of comments. In your third picture of the upside-down control box, I'd recommend a better strain relief on the power cord. It looks like it can move. A quick solution is the type of tie-wrap that has an eyelet with a screw hole in it for mounting. Those plastic cable clamps you're using are a bit flimsy in my experience, especially for something carrying line power.

I also suggest you buy a few rolls of different colors of vinyl tape (usually a couple of bucks a roll and they last a long time) and use them to color-code the wires. Then hand-draw a schematic with the color codes indicated on the wires. You or someone who has to maintain or fix your creation years down the road will appreciate that. Trust me, I hide my own Easter eggs, so I know. Oh, make sure the box includes the schematic. The best is to affix it to the inside of the back panel if there is one. Or, just put it in a plastic ziplock bag and toss it in before putting the back panel on.

If you didn't already, use either split or star lockwashers (I much prefer star lockwashers for electrical connections) under all your screw connections and tighten them snugly. They they'll never come loose. Also make sure to have a plain flat washer between the solderless terminal and the lockwasher (they're not mandatory, but they can help keep things from turning when tightening).

I assume you move the transformer/control unit off at a distance where it has no measurable effect on your EMF readings? If you really wanted to get anal, you could get some sheet mu-metal and surround the transformer to reduce the extraneous field. Since it sounds like you're only measuring line-frequency AC stuff, you could probably just use some steel sheet metal, as you wouldn't care about any remanent DC magnetization (but mu-metal is a much better shield material).