A multirange meter can be constructed in two units, the first containing the 0-1mA meter movement with switches to select various shunt and series resistors to give six d.c. current ranges up to 1 amp and eight d.c. voltage ranges up to 1000 volts. An internal battery provides an ohms range readable up to 200,000 ohms which corresponds with the first division of the meter (0.02mA)
An Add-on unit contains a meter rectifier with associated switched series resistors to give four a.c. voltage ranges up to 100 volts while additional shunt and series resistors (Ra and Rb) extend the range to 10A and 5Kv. When using the add-on unit the main instument is set to measure 1mA FSD and the add-on unit is connected to its terminals by its lugs.
The series resistors are 1% tolerance high stability types while the shunts are made of lengths of Eureka resistance wire. The wire used on electric fire elements is ideal for the lower value shunts. The values have been calculated for a meter resistance of 60 ohms internal resistance but would need modifying for other values. In any case, the precise value of each shunt should be adjusted experimentally to give the correct reading against a meter of known accuracy.
To calculate Rb: Rb= 1000V/I
Where I is the FSD of the meter and V is the desired voltage range
To calculate Ra: Rs= Rm/n-1
Where Rm is the meter resistance and n is the scale multiplication factor.
For example: If a milliammeter of 10 ohms resistance and a FSD of 1mA is to be used to measure 100mA, a shunt must be provided to carry the excess current, that is 100-1 milliamps,i.e 99mA. Thus the required shunt resistance is:
Rs= 10/100-1 = 10/99 = 0.101 ohms
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