Current is produced in a primary battery by the chemical action of a fluid known as an =ELECTROLYTE= upon two dissimilar metals or solids known as the electrodes. One of the electrodes, the negative, is usually made of zinc which is more readily attacked by the electrolyte than the positive electrode. As the metal of the negative electrode is dissolved and passes into the solution during the process of current generation, the electrolyte is also exhausted. The production of current is accompanied by the liberation of hydrogen gas from the electrolyte from which it is displaced by the zinc taken into solution. When the electrodes are immersed in the electrolyte, and the outer ends of the electrodes are connected with a wire, a current will flow from the positive electrode to the negative through the wire, and from the negative to the positive electrode through the fluid. It will be seen that to complete the circuit between the electrodes it is necessary that the current flows through the electrolyte. Electrical energy is actually generated in the primary battery by the chemical combustion of the negative electrode in the same way that heat energy is developed by the burning of a fuel. By connecting the binding posts of the electrodes to the two wires of the external circuit, a current will flow through the circuit as long as the electrodes remain undissolved, or until the positive electrode is covered with hydrogen gas bubbles. The bubbles of gas tend to insulate the positive electrode from the electrolyte or fluid, thus breaking the circuit through the fluid, and stopping the flow of current. This action is known as polarization. When a battery is polarized, the only remedy is to disconnect it from the circuit and allow it to rest or recuperate. The greater the current drawn from a battery, the more rapid the polarization, and it is evident that if the battery is to be used for long periods, polarization must be eliminated, or the current must be considerably reduced in volume. A battery that delivers a small current has a much greater capacity in ampere hours than a battery that has a higher rate of discharge. The greater the discharge rate the longer must be the rest periods. A battery that is designed for continuous service, or for delivering heavy currents of long duration, is called a closed-circuit battery. Polarization is eliminated in closed circuit batteries by various methods, the usual methods being to place some substance in the electrolyte that will destroy the hydrogen film; or by packing some solid oxidizing material around the positive electrode that will absorb the hydrogen; or by making the positive electrode of some material that will destroy the hydrogen as soon as it is developed.