Voltmeters should always be connected in parallel within a circuit because it measures what is known as the Potential Difference between the two points (see Fig 1). This potential difference or P.D is the voltage measured in volts (symbol V).
Analogue voltmeters, like ammeters have a limited scale such as 0-15V. It is important not to exceed the top limit as damage to the electromagnetic coil may result. Millivoltmeters (measuring in mV) and microvoltmeters (measuring in µV) measure tiny differences in voltage whereas some school voltmeters may go up to 200V and beyond.
Some meters, known as centre-zero voltmeters have the zero point in the middle of the scale. The needle in these devices can sweep in either direction to give a negative and positive reading.
Digital voltmeters usually have an LCD display and most that are used in school have a usable range of around 0-25V. They can be connected either way round unlike analogue meters and as such if connected the wrong way round they will display a '-' before the value.
Although digital meters have mostly superseded their analogue cousins, sometimes teachers require students to read straight from an analogue scale. Also in some applications such as electromagnetic induction, it is important to see the needle of an analogue meter sweep in either direction for this application, centre-zero meters are normally required.