496 MAGNETISM AND ELECTRICITY solution, while another metal in solution accepts electrons and goes out of solution. It is this chemical change that supplies the energy for the production of the electric current. A simple electric cell consists of strips of copper and zinc immersed in sulfuric acid. These two strips of metal are called the electrodes, while the acid solution, or any other solution that will conduct the electric current, is called the electrolyte. It has been found that an electric current may be produced by any of the following combinations of electrodes and electrolytes provided that there is differential chemical action. Electrode 1. Any two metals 2. One metal 3. One metal 4. Different concentrations of one metal Electrolyte in one electrolyte in two different electrolytes in two different concentrations of the same electrolyte in one electrolyte In every case there is a chemical reaction in which a stream of electrons is produced. About twenty years after the invention of the voltaic cell, a discovery was made which led to the invention of another generator of electric current, the electric motor. Before we take up the discoveries which led to the electric motor, we must pause briefly to learn a little about the electric current. An Electric Current Is Similar to a Flow of Water in a Pipe. In order that water shall flow in a pipe, the pipe must be of sufficient Fig. 235. The difference in water level is similar to the difference in electrical potential. size to carry the desired quantity of water, and there must be a pressure to maintain the flow of water. The pressure may be supplied by a tank filled with water at a higher level than the water outlet. If the water outlet is opened, the water will run out of the tank, and the flow of water will stop. This corresponds to the discharge of an electrical charge through a conductor. If the water level is maintained in the tank, then the water will continue to flow through the pipe at the same rate. This level may be maintained by a pump, which thus supplies the energy contained by the flowing water, instead of a
CURRENT ELECTRICITY 497 difference in water levels, there is a difference in the electric circuit, called a potential difference, which is maintained by the electric battery or the generator which supplies the energy to maintain a steady flow of electrons through the wire. The valve in the water pipe corresponds to the switch in the circuit. The requirements for an electric current are, therefore, a sustained difference of potential and a conductor. Fig. 236. The cell maintains a difference of potential and supplies the energy to operate a small motor when the switch is closed. Pipes offer resistance which cuts down the flow of water. Electrical conductors also offer resistance to flow. A small wire will not conduct so much current as a large wire for equal differences in potential, while many substances offer a high resistance to the flow of the current. In making diagrams of electrical circuits, a cell is designated by a long and a short vertical line parallel to each other. A straight line represents a conductor, while a zigzag line represents a resistance. The resistance of a circuit determines the amount of current that will flow under a given difference of potential. This is a simple statement of Ohm's law. The electric current, the difference of potential, and the resistance may all be expressed in familiar units: Current Amperes . (Ampere) Resistance Ohms . . (Ohm) Difference of potential . Volts . . (Volta) Difference of potential is also called voltage. An instrument that measures voltage is a voltmeter. It corresponds to the pressure gauge on a water line. The ammeter is the instrument designed to measure amperes, or current, and corresponds to a water meter (flow-meter type) which measures the flow of water in gallons per unit of time. Water meters, which measure the total amount of water flowing, correspond to coulometers, which measure coulombs. The unit of electric current, the ampere, corresponds to the flow of one coulomb per second. A coulomb per second corresponds to the flow of 6.30 X 1018 (6,300,000,000,000,000,000) electrons per second. It is the amount of current that will cause the deposition of 0.001118 gram of silver from a solution of silver nitrate in one second. The unit of resistance, the ohm, is the resistance offered by a conductor which conducts one ampere under a potential difference of one volt.