What is the device (meter) used to measure electric potential difference called?

$(a)$ Name an instrument that measures electric current in a circuit. Define the unit of electric current.
$(b)$ What do the following symbols mean in an electric circuit?
$(c)$ Draw a closed circuit diagram consisting of a $0.5\, m$ long nichrome wire $XY$,an ammeter,a voltmeter,four cells of $1.5\, V$ each,and a plug key.

In an electrical circuit,three incandescent bulbs $A, B$ and $C$ of rating $40 \, W, 60 \, W$ and $100 \, W$ respectively are connected in parallel to an electric source. Which of the following is likely to happen regarding their brightness?

$A$ piece of wire is redrawn,without change in volume,so that its radius is halved. Compare the new resistance with the original resistance.

The equivalent resistance between points $A$ and $B$ in the following electric circuit is:

Find the number of electrons transferred between two points kept at a potential difference of $20 \ V$ if $40 \ J$ of work is done.