One voltmeter has a resistance of $1000 \mathrm{\Omega }$ and another voltmeter has a resistance of $4000 \mathrm{\Omega }$. Which one of the two will you prefer to measure a potential · difference across a resistance of $500 \mathrm{\Omega }$ ?

Two identical circular loops, $P$ and $Q$, each of radius $r$ and carrying currents $I$ and $2I$ respectively are lying in parallel planes such that they have a common axis. The direction of current in both the loops is clockwise as seen from $O$ which is equidistant from the both loops. Find the magnitude of the net magnetic field at point $O$. 

Two identical circular loops, $P$ and $Q$, each of radius $r$ and carrying currents are kept in the parallel planes having a common axis passing through $O$. The dimlion of rurrent in $P$ is clockwise and in $Q$ is anti-clockwise as seen from $O$ which is equidistant from the loops $P$ and $Q$. Find the magnitude of the net magnetic field at $O$.