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The Philpott · 18th Jul 2019, 3:42 pm

Andy, i couldn't find the Model D glass tube i thought i had (on account of the fact it never existed..) but have sent you one from a Model 40 series for academic interest. Have a look at the thickness of the wire where it has pulled away from the tube- it is 1 thousandth of an inch thick, (25 micrometres)- a human hair is 3 thou. They managed to make these things to a resistance tolerance of around 0.2-0.3%...good even by modern standards.

When they made the slab and bobbin type windings they were always up against capacitance and inductance, hence the odd (usually patented or pat.pending) ways they wound them. I had one small bobbin that showed 12kohms then 14kohms when the polarity of the DMM leads was reversed- although i think this may have been down to the effects of internal corrosion. A resistor that dreamed it was a diode!

Dave

18th Jul 2019, 3:42 pm	#2215
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	Re: AVO Multimeter survey Andy, i couldn't find the Model D glass tube i thought i had (on account of the fact it never existed..) but have sent you one from a Model 40 series for academic interest. Have a look at the thickness of the wire where it has pulled away from the tube- it is 1 thousandth of an inch thick, (25 micrometres)- a human hair is 3 thou. They managed to make these things to a resistance tolerance of around 0.2-0.3%...good even by modern standards. When they made the slab and bobbin type windings they were always up against capacitance and inductance, hence the odd (usually patented or pat.pending) ways they wound them. I had one small bobbin that showed 12kohms then 14kohms when the polarity of the DMM leads was reversed- although i think this may have been down to the effects of internal corrosion. A resistor that dreamed it was a diode! Dave