Valhalla Scientific 4300C, Operation Manual

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74
Noisy Readings
When measuring very low resistances of highly inductive loads, care should be taken to reduce 60Hz
pickup at the input to the 4300C. The noise pickup is caused by the very high impedance of the load at
60Hz given by the formula:
Z = 377 x L
Where:
Z = Impedance in ohms
L = Inductance in Henries
Thus, with a 1000 Henry transformer the impedance at 60Hz would be 377Kohm, across which many
millivolts of noise may be induced by movement or by interference from other devices.
This effect may be reduced by using fully shielded cables such as Valhalla Option "K". It may also be helpful
(and will cause the settling time to be reduced) if the unused windings of the transformer can be short-
circuited during the measurement. This will significantly reduce the inductance of the winding under test,
and will also prevent the unused windings from producing dangerous voltages during charging and
discharging of the transformer.
When using the 20mV voltage range, the measurement may be subject to the effects of thermal EMF's.
This effect is seen as noise on the display. The best way to reduce this effect is to use a low-thermal lead
set such as Valhalla Option SL-48.