Dummy resistor

Figure 25.2 Constant-current source using a battery and series resistor, (a) Dummy (resistor) load (b) Norton s equivalent circuit for part a (c) electrolysis cell as the load.

As shown in Figure 22-13, the equipment required for a coulometric titration includes a source of constant current from one to several hundred milliamperes, a titration cell, a switch, a timer, and a device for monitoring current. Moving the switch to position 1 simultaneously starts the timer and initiates a current in the titration cell. When the switch is moved to position 2, the electrolysis and the timing are discontinued. With the switch in this position, however, current continues to be drawn from the source and passes through a dummy resistor that has about the same electrical resistance as the cell. This arrangement ensures continuous operation of the source, which aids in maintaining a constant current. 

With the electrochemical instrument turned off, disconnect the cell and replace it with a 10 kfl resistor (dummy cell), with the reference and counter electrode leads connected together on one side of the resistor and the working electrode lead connected to the other side. Scan from +0.5 to -0.5 V with the sensitivity of the instrument set for currents of about 100 /xA, The resulting scan should be a straight line that intersects the origin with maximum currents of 50 fxA. 

In practice, two of the dummy capacitors C could equally be replaced by resistors in adjacent arms of the bridge with provision made for balancing the bridge initially. 

When trying to eliminate or minimize the effect of 60-Hz sources, it is sometimes useful to use a dummy source, such as a fixed 100-kS2 resistor attached to the electrodes. By employing a dummy source as one of the input signals, the output of the differential amplifier represents only contributions from interfering sources. If noise can be reduced to an acceptable level (at least by a factor of 10 less than EEG signals), it is likely that uncontaminated EEG records can be obtained. 

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