why there is series resistance in voltmeter?

 

Because basically a moving coil meter is a current operated device, the series resistor tailors the scale in order for it to indicate voltage according to the value of the resistor.
Max.

 

A very high series resistance is desired, since ideally you don't want the meter to draw any current and thereby distort the voltages you are reading. The more current it draws from your circuit under test, the less accurate your meter reading will be.

 

A moving coil volt meter is usually indicated in Ohms/Volt, this figure is ideally as high as possible depending on the FSD current of the meter, this is the desired value mentioned in wanyeh post.
Max.

 

Better analog VOMs are rated at 20kΩ/V or better, like my Heathkit MM-1, cheaper ones down to 2kΩ/V.
Digital meters generally are fixed input restance, Harbor Freight freebees @ 1MΩ, better ones 10MΩ & up.