The joulemeter/wattmeter measures the total power dissipation in the load, including that of any
meters connected in the load circuit.
The instrument records the product of the simultaneous values of current and potential difference
(voltage across the load) at every instant.
Moving iron meters record effective values of current and p.d. and so, in d.c. and a.c. resistive
circuits, give readings consistent with those of the joulemeter/wattmeter.
Moving coil meters, however, record average values and when they are used the nature of the
supply must be taken into consideration as follows:
1. 'Pure' d.c. sources, e.g. accumulator, dry battery: the meter readings represent effective
values.
2. Mains-driven d.c. power supplies (full-wave rectified):
a) Unsmoothed. Both current and voltage readings must be multiplied by 1.11 to obtain
effective values.
b) Smoothed. The degree of smoothing, and hence the approximation to pure d.c., often
depends on the current drawn by the load.
3. a.c. Sources:
A.C. shunts and multipliers fitted to moving coil meters provide full wave rectification. They are
usually adjusted to record effective values for sinusoidal waveforms. Where current transformers
are incorporated, shunts are calibrated at frequencies of 50 - 60Hz. Meter readings DO NOT
necessarily represent effective values for other waveforms or widely different frequencies.
Initial procedure
Switch on the instrument and allow a short time for transient counts to die away, then reset the
display.
Connect the load and the supply. For energy measurement it is often convenient to include an on-
off switch in the supply circuit.
Select the appropriate ‘load current’ setting (if known) otherwise set up the experiment to use the
maximum current, set the ‘load current’ to high, then turn the load current knob down until the over
current LED lights, then turn back one step.
If the over current LED lights during an experiment, turn the ‘load current’ control to a higher
setting, or change the experiment setup.
Move the display switch to show energy or power, this may be done at any time without affecting
the readings. If both energy and power are required, connect a 10V moving coil meter to the
power sockets.
Note:
1. Changing the <+">load current<-"> setting, immediately resets the meter.
2. If spurious counts are to be avoided, the mains switch on the instrument should NOT be used
to start or finish a reading.
3. Within the limits of accuracy of the instrument slight drift may occur, giving rise to occasional
counts or slight offset to the wattmeter zero.
4. If earthed supplies and measuring equipment are used, care must be taken to ensure that
short circuits do not occur as the lower Supply socket, the black power and black pulse sockets
are connected inside the unit.
