Electricity, it is also conceded, is without weight, and, while it is without doubt, one and the same, it is for convenience sometimes classified according to its motion, as: 1. Static electricity, or electricity _at rest_; 2. Current electricity, or electricity _in motion_; 3. Magnetism, or electricity _in rotation_; 4. Electricity _in vibration_ (radiation). Other useful divisions are: 1. Positive; 2. Negative electricity; 3. Static; 4. Dynamic electricity. =Static Electricity.=--This is a term employed to define electricity produced by friction. It is properly employed in the sense of a static charge which shows itself by the attraction or repulsion between charged bodies. When static electricity is discharged, it causes more or less of a current, which shows itself by the passage of sparks or a brush discharge; by a peculiar prickling sensation; by a peculiar smell due to its chemical effects; by heating the air or other substances in its path; and sometimes in other ways.[1] =Current Electricity.=--This may be defined as the quantity of electricity which passes through a conductor in a given time--or, electricity in the act of being discharged, or electricity in motion. An electric current manifests itself by heating the wire or conductor; by causing a magnetic field around the conductor and by causing chemical changes in a liquid through which it may pass. =Dynamic Electricity.=--This term is used to define current electricity to distinguish it from static electricity. =Radiated Electricity.=--Electricity in vibration. Where the current oscillates or vibrates back and forth with extreme rapidity, it takes the form of waves which are similar to waves of light. =Positive electricity.=--This term expresses the condition of the point of an electrified body having the higher energy from which it flows to a lower level. The sign which denotes this phase of electric excitement is +; all electricity is either positive or negative. =Negative Electricity.=--This is the reverse condition to the above and is expressed by the sign or symbol -. These two terms are used in the same sense as _hot_ and _cold_. =Atmospheric Electricity= is the free electricity of the air which is almost always present in the atmosphere. Its exact cause is unknown. The phenomena of atmospheric electricity are of two kinds; there are the well known manifestations of thunderstorms; and there are the phenomena of continual slight electrification in the air, best observed when the weather is fine; the Aurora constitutes a third branch of the subject.