Galvanometer zero line

In polarography it is a convention to denote as cathodic those currents, corresponding to a reduction process, occurring above the galvanometer zero line. Currents flowing in the opposite direction, i.e. causing deflections imder the zero line, are called anodic (Fig. 3). 

Fig. 22. Mixtures of different ratios of depolarizers reduced at more positive (A) and more negative (B) potentials or giving anodic waves at more negative (C) and more positive (D) potentials. Cathodic waves right, galvanometer zero line at the bottom. Anodic waves left, galvanometer zero line at the top.

When both the titrated substance and the titrant are polarographically active at a chosen potential, a V-shaped titration curve is observed (Fig. 48). When the titrated substance shows an anodic wave and the titrant a cathodic wave, the anodic wave decreases and the cathodic wave increases during the titration, and a titration curve crossing near the galvanometer zero line is observed (Fig. 49). 

Polarographic waves are observed only in those molecules or ions— organic or inorganic—which are capable of electro-reduction or -oxidation at the dropping mercury electrode. In cases of electroreduction the waves and currents are termed cathodic, and in cases of electro-oxidation, anodic. The borderline dividing the cathodic from the anodic currents is the zero line of the galvanometer, corresponding to the currentless state. Cathodic waves are then represented above, and anodic waves below, this zero line. 

Galvanometer with Variable Series Resistance.— As shown above, galvanometers, especially those of low resistance, are usually calibrated to read correctly for a definite line resistance. Suppose an indicator is desired for operation on a line of which the resistance changes from practically zero to 10 ohms. The instrument is calibrated to read correctly for a line resistance of 10 ohms. Located in the case of the galvanometer and in series with the line is a variable resistance from zero to 10 ohms. As the resistance of the line increases the variable resistance is decreased by manual adjustment so that the sum of the line resistance and the variable series resistance always equals 10 ohms. The dial of the variable resistance is graduated to read the amount of resistance cut out of the circuit. Hence, it should be set at the resistance of the line and couple. 

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