The quantity of flow in an electric current is called _amperes_ and the pressure is designated as _volts_. CONDUCTIVITY.--All metals conduct a current with greater or less facility. Silver is the best conductor, followed by copper. German silver offers a great resistance, and many alloys offer greater or less opposition to the flow. RESISTANCE.--The length of a wire also serves to check the flow, and this may be overcome by enlarging the size of the wire, or by increasing the pressure, or voltage. GENERATING ELECTRICITY.--A current may be generated by a dynamo, or by means of cells. The dynamo derives its motion from an engine, which turns, what is called, the _armature_ past a number of magnets, called the _field_. The armature contains a series of wire wrappings, extending around from end to end, and the field is composed of metallic heads, each carrying a coil. MAGNETIC FIELD.--When these coils have a current flowing through them the heads become magnetized, and have what is called a _magnetic field_ surrounding them and extending out some distance, and the armature coils pass through these magnetic fields. As these wires cut the lines of force in the magnetic fields, a current is set up in the armature, and as the armature windings are connected up with the lead and the return wires which transmit the current, it will be seen that the strength, or pressure of the current depends on the speed of the armature movement. BATTERIES.--The other method of generating a current is to use a jar of _electrolyte_, a liquid which may be either an acid or a salt solution. If certain metals which are opposite to each other, are placed in this solution, a chemical action takes place, which results in producing current, and this may be shown by connecting together the two metals by a wire outside of the jar. METALLIC COUPLES.--Within the jar the solution serves as the conductor between the two metals. Copper and zinc are two good metal couples, in which zinc is the positive, and copper the negative. As zinc is readily eaten away by the action of the electrolyte, carbon is used instead. WHAT DETERMINES VOLTAGE.--Each cell with the two metals, will furnish approximately two volts. It is immaterial whether the cell contains a pint or a gallon of liquid, or what the size of the plates may be. In any event the pressure will not be greater than two volts. CONTROLLING AMPERAGE.--But the metal plates may be made very large, or have a great surface in each cell. The greater the surface the greater the amperage, so that while each cell has only two volts, it may have a very small amperage, or it may have two, five, ten, or even more amperes flowing therefrom.