NEWS AND RECENT ADDITIONS

                                    ELECTRODES

Recently added is a new section detailing the construction of low cost electrodes made from pencil leads. Use of these in place of the original commercial (RS) electrodes saves some £50 in the overall cost of the apparatus. They are easy to construct, work very well and are robust & reliable in use.

                                 ELECTRONICS

Following a re-examination of the use of a 741 OPAMP in place of the long-tailed pair thatwas originally used as the differential amplifier, the 741 has been found to be better, particularly in the removal of common mode signals that (inevitably) arise from transients on the input square wave signal. A lower balance null voltage* is achieved and the noise is lower.

* Never quite zero owing to residual ac components that arise from electrode polarisation (see next section). NEW CIRCUIT SHOWN IN ELECTRONICS SECTION

The effects of electrode polarisation can be reduced by increasing the frequency of the input square wave. By changing just one resistor the frequency has been raised from 10.3 kHz to 27.8 kHz. A transient on the rising edge of the input waveform is effectively attenuated by the use of the 741 (above).

CALIBRATION, Linearity etc. [see sub-page to RHS of Results]

Following detailed studies of the above new information is being added to the calibration section. Guidelines have been evolved for allowing very sensitive measurements over a 100,000-fold range of conductivity.