Electrode

ELECTRODE, a device used for conducting electric current to the body or for drawing it away from the latter. An electrode carrying a positive charge is called an anode; a negative one is called a cathode. Two main types of electrodes are distinguished: 1) ordinary metal electrodes and 2) reversible, non-polarizing electrodes. The advantage of the first type of electrode is the simplicity of their manufacture, the possibility of achieving good contact with the object under study, as well as the absence of restrictions on the choice of their form. A significant disadvantage of these electrodes is, on the one hand, their oxidizability, and on the other, the phenomenon of polarization when electric current passes through. The latter consists in the appearance at the point of contact between the electrode and the object under study of a contact potential difference that changes over time. The magnitude of the latter is determined by the change in the electric charge of the metal located at the boundary of two phases: metal-living tissue. This change occurs on the basis of electrolytic processes arising on the surface of the metal. The direction of polarization currents is generally opposite to the direction of the primary current. Due to the superposition of these currents on each other, one has to measure the difference between both currents and thus sometimes introduce a significant error into the measured quantities (voltage or current strengths). It should be noted that the magnitude of the polarization current changes over time and increases not directly proportionally, but along a certain curve, thereby causing a progressive increase in successive errors in measurements. Electrodes of the second type, reversible or non-polarizing, were proposed with the aim of eliminating this significant disadvantage of metal electrodes. In this case, the contact potential difference is largely eliminated by using a combination of a metal and a saturated solution of its sparingly soluble salt. Under such conditions, the passing current decomposes the salt into a positively charged metal cation and a negatively charged acid residue-anion. The cation moves to the cathode and settles on it without changing its chemical composition; the anion combines again with the metal of the anode to form the corresponding salt. No polarization occurs in this process. Metal electrodes are usually used for electrotherapy and electrophysiological diagnosis purposes. At the same time, the most commonly used are copper or lead button electrodes covered with suede or cloth: the normal electrode of Stinzinger (surface of contact=3 cm²) and the normal electrode of Erb (surface of contact=10 cm²). For some physiological purposes, in cases where polarization is not of essential importance, metal electrodes in the form of platinum and silver wires of various diameters, mounted on ebonite plates, are used. Non-polarizing reversible electrodes are used mainly in physiology, bio- and physico-chemistry. Various modifications exist. The most common are the following: the normal hydrogen electrode, consisting of platinum covered with platinum black and saturated with gaseous hydrogen. At a certain pressure and a certain concentration of hydrogen ions, this electrode gives a quite definite potential. It is usually used in determining the concentration of hydrogen ions (H). The normal calomel electrode (or half-element), consisting of mercury covered with calomel and washed with a normal KCl solution, is often used to determine the potential of another unknown electrode or serves as a standard. For physiological research, the following non-polarizing electrodes are very frequently used: zinc electrodes, consisting of an amalgamated zinc rod immersed in a saturated solution of zinc sulfate. To protect the tissues from the damaging effect of Zn ions, the end of the glass tube with a saturated ZnSO₄ solution is filled with cotton (Pinselelectrodes) or covered with a cap made of white clay mixed with physiological solution. Electrodes proposed by Lapicque and d'Arsonval have recently become widespread among Soviet physiologists. These electrodes consist of a silver wire coated with a layer of AgCl or chlorinated in a corresponding manner. This wire is immersed in a Ringer's solution or NaCl. Non-polarizing microelectrodes, first constructed by Ettisch and Peterfi, consist of a glass capillary tube bent at a right angle and drawn out into a thinner capillary at one end. The tube is filled with a 2% solution of agar-agar prepared in a N/₁₀ KCl solution. A chlorinated silver wire is inserted into the capillary. These electrodes are used for measuring potentials along microscopic sections of tissues.