An introduction to what voltmeters measure and how they are connected in circuits.
Apparatus and materials
Water circuit board
Variable voltage supply (0-12 V, AC or DC as required by pump)
Leads, 4 mm
Health & Safety and Technical notes
The water circuit board should be set up vertically. The electric motor, which drives the water pump, should be connected to the terminals of the variable voltage supply. The tubes should be filled with water: a little fluorescein or a few drops of methyl orange can be added to make the water more clearly visible. The water is conveniently poured in at the funnel. The pump will drive water round the circuit of glass tubing attached to the board, the pressure being dependent on the voltage applied to the motor.
At one point, the tube divides. The two sections represent different resistances: one tube has a much finer bore than the other. Clips enable one or other or both sections to be opened at once: thus the effect on the current of different 'resistances ' can be seen.
The pressure gauge consists of a U-tube connected as illustrated and filled with coloured water.
Where there is a break in the circuit, the funnel catches the water flowing down from the tube above. The rate of flow of water is apparent and this indicates the current. Alternatively, if there is a pool of water in the funnel, the faster the flow of water the more rapid the swirling motion in the funnel. A small piece of cork floating on the water in the funnel acts as an indicator of the rate of swirling. This shows the current.
You can show the flow of water more clearly like this: push a large sewing needle into a small cube of expanded polystyrene so that the cube is half way along it. Put one end of the needle into the tube at the base of the flow meter so that the polystyrene floats on the water. If the funnel is inclined a little then the water will swirl more easily.
a Initially, it is not advisable to draw attention to the analogy with an electric circuit. Start by showing how the rate of flow of water (the current) depends on the pressure pushing it. To do this, change the speed of the pump by altering the voltage of the supply. The U-tube indicates the pressure difference across the tube at the top of the board.
b Go on to show the effect of changing the bore of the tube at the top. A narrower tube allows a smaller current to flow, provided the pressure difference is kept constant. Emphasize the need to adjust the pumping rate to ensure that the pressure difference is constant.
c Now you can discuss the analogy with an electric circuit. The flow of water represents the current, and must be conserved around the circuit (unless you have a leak!). The pump represents the battery pushing it round. Greater pressure difference corresponds to greater voltage (or potential difference). A narrower tube corresponds to greater resistance.
d Test your students' understanding by asking them to predict the effect of having two narrow-bore tubes in parallel with each other at the top of the board. (The current should double, compared with a single tube.)
e Once students have used other components, they might be able to suggest how the water circuit analogy can be modified to represent diodes (one way valves) and capacitors (reservoirs).
This experiment was safety-checked in November 2006