why voltmeter and resistence are parallel in ohm's law.

Never mind Ohm's Law.  A voltmeter measures the difference in electrical potentialbetween two points.  So if you want to measure the difference in electrical potentialbetween two points, you touch one probe of the meter to each point, and somehowthe internal guts of the meter are able to sense the difference in electrical potentialbetween the two points. If the two points have non-reactive impedance (a resistance) between them, then the difference in electrical potential between the two points, as indicated by the meter,will be close to (resistance between the two points) x (current flowing between them).