Question

why do we multiply wattmeter constant with wattmeter reading? what is the significance of wattmeter constant or multiplying factor?

Answer 1

Depends if your measuring real or apparent power.

Answer 2

can u be still clear....

Answer 3

wattmeters has got different range of readings ,so to get the actual power ,you need to multiply it by a factor called as the multiplication factor.Generally these factors are printed below the watt meter by the manufacturers themselves and also these are standard for a given value of I and V , so it varies with different range and ratings.
like for 10 A M,L,C,V are at their usual postion and MF is 4
so for 20 A it will be 8
theoritically
Multification factor =(voltage range*current range*pf)/Max scale deflection