into water and attached to the poles of a battery, they become polarized; that is, the anodic plate becomes covered with bubbles of oxygen, the kathodic with bubbles of hydrogen. If now the plates are connected with the terminals of a galvanometer, it is found that a polarization current is set up from the hydrogen through the liquid to the oxygen (i. e., in the opposite direction to the battery current), and that this polarization current may last a long time if the plates are large enough. This property has been made use of in the construction of Plante's storage cell and Trouvé's Electrical Polyscope. Accessory Apparatus.-The Galvanometer is an instrument used to show the presence, strength and direction of a current of electricity. The Rheostat is so constructed as to allow an accurate gradation of current resistance when it is placed in the circuit. The Selector or Cell-enumerator is an arrangement by which any desired number of cells may be at once brought into circuit. The Polarity Changer or Commutator affords a means of instantly changing the direction of the current and then reversing the poles. The Interrupter (for the galvanic current) is usually contained in the handle of the electrode; a button is pressed upon, and this by means of a spring makes and breaks the contact. Electrodes are the direct means of applying electricity to the body; they are of very various forms, as shown on pages 45 and 47. Disks of carbon covered with waste leather are preferable to those of metal covered with sponge. Rheophores are the wires connecting the electrodes with the battery; they should be of ordinary telegraph wire insulated by a rubber coating. |