[ARC5] Carbon Comp Drift

[Richard Knoppow]

    At some point the resistance of the measuring circuit becomes 
important. If you zero the VOM with the same leads used to 
measure resistance the adjustment will tend to eliminate the 
internal resistance of the meter.
    For really low values, and really for lower than about 100 
ohms but certainly for less than about 10 ohms, a Kelvin double 
bridge is one of the ways of getting accurate measurements. This 
bridge has four leads, to current leads and two voltage leads. 
Essentially it measures the voltage drop across the resistor as 
compared to an internal standard. You can do the same thing 
without a bridge but you need to be able to measure the current 
accurately as well as the voltage drop across the resistor. For 
most resistors found in radio sets a plain ohm meter as found in 
a good quality VOM is sufficient. You can calibrate a VOM with a 
precision resistor, but, remember the meter is indicating the 
resistance at its input leads just as a bridge measures at the 
bridge terminals. The advantage of the Kelvin bridge is that it 
moves the effective measuring terminal to the junction of the 
current and voltage leads and corrects for their resistance.
     I have a Triplett 630A, very similar to the Simpson 260. I 
used this thing constantly in favor of my Tek DMM. The DMM has 
better specs but I much prefer an analog meter for old equipment. 
The Triplett specs are acutally quite good.
     When I did a search for metal oxide resistors I found Mouser 
lists lots of carbon comp resistors. I was looking for something 
else so didn't note details, however, they are evidently available.
     I suspect that old carbon comp resistors that have been 
stored in a dry place are probably as good as new. However, 
moisture does damage them as does heat.
     BTW International Resistance Company used to advertise 
"metalized" carbon comp resistors. I have never been able to find 
out what exactly they did. I am sure a patent exists somewhere 
but I have been unable to find it. IRC resistors are generally 
larger than Ohmite and others and have a rough tree-bark sort of 
finish that IRC claimed dissipated heat better. They also claimed 
they were lower noise than other CC resistors.  I hardly ever see 
them in old gear although the catalog prices were about the same 
as Ohmite, Allen-Bradley and others.

On 3/2/2019 5:11 PM, Robert  Eleazer wrote:
> The problem I have seen with reading the resistance of very low 
> valued resistors is that VOM's do not seem to be designed to read 
> that low.  Even trying to get a digital ohmmeter that will zero 
> on its lowest range is a challenge.  I have a Simpson 260 in a 
> heavy protective roll-top case I should dig out, come to think of 
> it.
> I recall an article of many years ago on how to build a 
> "Lo-Lo-Lo" ohmmeter, suitable for the low ranges.
> Wayne
> WB5WSV


-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL