Page 834 - Basic Electrical Engineering

The total circuit resistance is R + R m
4

= 9950 + 50

= 10,000 Ω

Similarly, the values of R , R , and R can be catantated for higher voltage
1
3
2
ranges.
The sensitivity of a voltmeter is expressed as the ratio of the total circuit
resistance to the voltmeter rating, i.e., in terms of ohms per volt.

11.9.3 Moving Iron Instruments

Moving iron instruments are used to measure current and voltage. They are
made as ammeters and voltmeters and can measure both ac and dc currents

and voltages, respectively. Two types of moving iron instruments are
manufactured. One type is called attraction type and the other type is called

repulsion type.

Attraction-type MI instruments: In this type of instruments there is one fixed
coil and one moving iron. The fixed coil is wound on a former. The current to

be measured or a current proportional to the voltage to be measured is
allowed to flow through this fixed coil. A disc made of iron is attached to the

spindle. A pointer is also attached to the spindle. Current passing through the
fixed coil produces a magnetic field inside the coil. This magnetic field

attracts the piece of iron which is free to move around the spindle. When the
iron disc gets attracted it moves inside the coil. When this happens, the

pointer moves over a graduated scale. The movement of the iron piece is
always from the weaker magnetic field that exists outside the coil towards the