not tallking to that sort of "quallity" . I mean , can't I buy simple fine quality switch ( link below)
and wound my own resistors from nichrome ?
Sure, but there are big problems.
1. Nichrome, like the one you referred to, changes by 2% going from 20C to 100C. That is much worse then many off-the-shelf resistors. Genrad probably used selected batches of magnanin or Evanohm wire to make those resistors.
2. You have to get different gauges wires for different resistance ranges. You will probably have trouble finding wire thin enough to make the 10K resistors. The wire used in the decade box is coated while most of the cheap nichrome wire is uncoated so you cannot let adjacent turns touch. The formers Genrad use is ceramic, so they may even be able to anneal the wires to improve stability.
3. It is very hard to get switches that are any good. The ones you have found are probably rated at something like 20 milliOhms resistance when new, and they don't have the self-cleaning property of the solid silver alloy switches. You can parallel the poles, so that switch you found is probably a good find. You might be able to get the contact resistance down to 5 milliohms. Whereas the switches in Dave's decade box tend to have a very consistent resistance, your $10 switch may have contact resistance that can vary each time you change the position, particularly at very low currents. You can probably make a good decade box of up to 0.01% accuracy in the upper ranges if you use commercial high stability resistors. However, you may be looking at a price of over $60 a decade for decent 1/2W precision low drift high stability resistors.
Home
Help
Search
Login
Register
